Make groups&items propagations twice faster, do not join groups table to determine group memberships, do not mark new items for propagation + testing-related fixes

[zenovich]

1. Make groups&items propagations twice faster:

• introduce a new column groups_groups.is_team_membership which equals to 1 when a parent group is a team, set its value automatically on groups_groups creation,
• move the 'processing' state into a new temporary table,
• always filter *_propagate tables by exact values of ancestors_propagation_state,
• perform covering index lookups on groups_groups instead of index lookups (create an index for this).

2. Do not join the groups table to determine if group memberships (use the new groups_groups.is_team_membership column instead).
3. Do not mark new items for propagation, only call the items propagation in itemCreate if at least one items_items row is created.
4. Disallow modifying groups.type and groups_groups.is_team_membership.
5. Change the DB username for migration-tests to match the definer of triggers in the dump file.
6. Sort all the data returned by currentUserDataExport & currentUserFullDataExport to make tests stable.