Move TinyMCE code into its own module

[GuySartorelli]

git TinyMCE 7 changed its license to GPLv2+, which isn't compatible with our licenses, so we need to swap to a new editor.

Regardless of whether a new editor is available in CMS 6.0.0 or in a future minor release, we want to:

1. Make sure TinyMCE 6 is still available for people who aren't able to swap away from it immediately
2. Move the TinyMCE 6 code into its own module that can be included or excluded from projects easily.

Notes

• https://github.com/silverstripe/silverstripe-htmleditor-tinymce has been created for this

Acceptance Criteria

The below all apply for CMS 6 only

• All TinyMCE related code (JavaScript, PHP, YAML, localisations etc) from all supported modules is moved into a single new repository
• Most (maybe all) localisations are not in transifex, though there may be some that are for plugins that need to be migrated, including in transifex itself - these must be copied not moved so as not to break CMS 5)
• The default "cms" editor config is provided in the new module
• TinyMCE plugins that rely on a specific module (e.g. "link to a page") are only applied if the corresponding module is installed
• There is configuration to disable the "cms" editor config from being set (e.g. if someone has two editor modules installed, they can pick which one controls the "cms" editor config with a simple YML config variable)
• When this module is installed, the behaviour is identical to the CMS 5 behaviour
• This module is installed as part of silverstripe/installer by default for CMS 6 (not recipe-cms)
• This module is not a dependency of any of our modules
• If no WYSIWYG module is installed, DBHTMLText will auto-scaffold an HTMLEditorField rendered as a textarea tag
• The changelog makes it very clear that this (or an alternative if it exists) must be included as a dependency by projects when upgrading from CMS 5 to CMS 6.

PRs

CMS 5

• API Deprecate API that will be removed in CMS 6 silverstripe-cms#3064
• API Deprecate API that will be removed in CMS 6 silverstripe-framework#11630
• DOC Document API deprecations for TinyMCE migration developer-docs#713

CMS 6

• ENH Update externals list for TinyMCE migration webpack-config#80

Important

Merge the above first and then reassign to Guy - the dist bundle in the tinymce module won't be buildable by CI until that's done

kitchen sink CI run with TinyMCE (The TinyMCE unit tests are in g2)
kitchen sink CI run with all PRs except TinyMCE (subsites and elemental behat should both fail, as they have explicit tests that can only be satisfied by tinymce right now)

• NEW Migrate TinyMCE code and config to new repo #1
• API Remove code which is being migrated to a new TinyMCE module silverstripe-framework#11617
• API Remove code which is being migrated to a new TinyMCE module silverstripe-admin#1902
• API Remove code which is being migrated to a new TinyMCE module silverstripe-asset-admin#1544
• API Remove code which is being migrated to a new TinyMCE module silverstripe-cms#3059
• ENH Remove unnecessary TinyMCE-specific code. silverstripe-reports#209
• Various changes to support TinyMCE moving silverstripe-behat-extension#300
• MNT Add new context to behat config silverstripe-elemental#1334
• ENH Add new tinymce module supported-modules#54
• DOC Document new tinymce module developer-docs#691
• ENH Add TinyMCE docs doc.silverstripe.org#322

Note: Upon reviewing the behat tests in elemental, it's clear they are explicitly for testing WYSIWYG functionality within elemental blocks and cannot be moved elsewhere.

Still needs to be done

• Add new repo to packagist
• Run module standardiser across new repo
• Reduce the number of exceptions in phpcs.xml.dist
• Any other steps from setting up a new repository (private confluence doc)
• DEP Add new TinyMCE module as an explicit dependency. silverstripe-installer#386 (do not merge until the repo has been added to packagist)
• MNT Add new TinyMCE module test suites. recipe-kitchen-sink#87 (merge after installer PR)

[GuySartorelli]

I'll make the requested changes once the CMS 5 PRs are merged. For now we should focus on coming to an agreement on #1 (comment) which is the only thing holding those PRs back from being merged.