avoid ''NoneType' object has no attribute 'exists'' error when purging models
fix my change [email protected], wrong lines replaced...

    raw SQL query (but never read afterwards). Workaround for
    lp:1277899

[FIX] Preserve dangling workflow table which is in use

[RFR] Group models per table when detecting columns to purge
      to prevent problems with models sharing the same table

[ADD] Allow purging of dangling data entries

[FIX] Data purging now working

[IMP] Docstrings

[FIX] Label
[FIX] Catch attempt to unlink field from nonexisting model

[RFR] Flake8

[CHG] database_cleanup: move description to README.rst

[IMP] order wizard lines by name

[IMP] deal with modules whose models can't be loaded

[IMP] double quotes for docstring

[FIX] use exists query instead of huge in list

[IMP] hide unnecessary buttons in wizard II

[IMP] readability

[FIX] cope with purging nonexisting models

[IMP] adaptto upstream changes

[ADD] test purging modules

[ADD] test purging tables

Fixes #505

[FIX] database_cleanup reloads the registry

which has weird side effects during testing. Take care
database_cleanup's tests don't mess up the following tests

* [FIX] database_cleanup: Isolate build
* Isolate `database_cleanup` into its own build in Travis file to fix #689

* [FIX] database_cleanup: Remove KeyError assertion
* Remove KeyError assertion in tests due to PR in comment being merged

* [ADD] allow creating missing indexes

* [FIX] tests; installation

* [ADD] allow purging properties

* [ADD] missing file

* [ADD] test purging properties

* [ADD] missing parent_id for menu entry

* [FIX] don't delete too many and wrong properties

Using new base model inheritance.

[FIX] don't try to uninstall uninstalled modules

[DEL] weird code

[FIX] actually cleanup where we can

Update version number
Use correct import
Update .pot to match new syntax
Adapt icon to fontawesome
Add models description
Rename field external id to new syntax <model>__<field>

cf discussion on #1408

To match the new database_cleanup.pot

Following odoo/odoo@54238c88e85d6e a Many2many may have no relation
This avoids having a None record in the list (which is problematic with the
NOT IN clause)
https://wiki.postgresql.org/wiki/Don't_Do_This#Don.27t_use_NOT_IN

Since 12.0, some virtual modules are created as an incentive to buy enterprise
Hide installation warning

Updated by "Update PO files to match POT (msgmerge)" hook in Weblate.

Translation: server-tools-12.0/server-tools-12.0-database_cleanup
Translate-URL: https://translation.odoo-community.org/projects/server-tools-12-0/server-tools-12-0-database_cleanup/

…d for uninstalling

Updated by "Update PO files to match POT (msgmerge)" hook in Weblate.

Translation: server-tools-12.0/server-tools-12.0-database_cleanup
Translate-URL: https://translation.odoo-community.org/projects/server-tools-12-0/server-tools-12-0-database_cleanup/

Currently translated at 100.0% (74 of 74 strings)

Translation: server-tools-12.0/server-tools-12.0-database_cleanup
Translate-URL: https://translation.odoo-community.org/projects/server-tools-12-0/server-tools-12-0-database_cleanup/pt_BR/

Currently translated at 51.4% (38 of 74 strings)

Translation: server-tools-12.0/server-tools-12.0-database_cleanup
Translate-URL: https://translation.odoo-community.org/projects/server-tools-12-0/server-tools-12-0-database_cleanup/zh_CN/

Currently translated at 100.0% (74 of 74 strings)

Translation: server-tools-12.0/server-tools-12.0-database_cleanup
Translate-URL: https://translation.odoo-community.org/projects/server-tools-12-0/server-tools-12-0-database_cleanup/zh_CN/

Currently translated at 100.0% (74 of 74 strings)

Translation: server-tools-14.0/server-tools-14.0-database_cleanup
Translate-URL: https://translation.odoo-community.org/projects/server-tools-14-0/server-tools-14-0-database_cleanup/es_AR/

Currently translated at 20.2% (15 of 74 strings)

Translation: server-tools-14.0/server-tools-14.0-database_cleanup
Translate-URL: https://translation.odoo-community.org/projects/server-tools-14-0/server-tools-14-0-database_cleanup/it/

No need to rollback here as TransactionCase does a safe rollback.

Needed for purging payment providers which recompute module_state

Signed-off-by pedrobaeza

Currently translated at 100.0% (193 of 193 strings)

Translation: server-tools-16.0/server-tools-16.0-excel_import_export
Translate-URL: https://translation.odoo-community.org/projects/server-tools-16-0/server-tools-16-0-excel_import_export/es/

Updated by "Update PO files to match POT (msgmerge)" hook in Weblate.

Translation: server-tools-16.0/server-tools-16.0-database_cleanup
Translate-URL: https://translation.odoo-community.org/projects/server-tools-16-0/server-tools-16-0-database_cleanup/

Currently translated at 80.8% (156 of 193 strings)

Translation: server-tools-16.0/server-tools-16.0-excel_import_export
Translate-URL: https://translation.odoo-community.org/projects/server-tools-16-0/server-tools-16-0-excel_import_export/it/

Currently translated at 100.0% (76 of 76 strings)

Translation: server-tools-16.0/server-tools-16.0-database_cleanup
Translate-URL: https://translation.odoo-community.org/projects/server-tools-16-0/server-tools-16-0-database_cleanup/es/