XWIKI-13967: Confusion between Supported Languages and Default Languages when multilingual is set to no
* Fixed a test that should have been updated with the fixes for XWIKI-13967

  * Fix the wrong expectation for NotificationEmailsIT tests when
    checking mail content (was containing bad code for users' avatars)

  * Provide a test page for the notification email velocity template
  * Fix a bug in diff_macros.vm
  * Minor fix in NotificationRSSDefaultPageTest

XWIKI-13967: Confusion between Supported Languages and Default Languages when multilingual is set to no (#2770)

* Added a CSS ruleset to remove the supported languages field from the UI when it's not useful.

* Replaced inline styling with regular style for the panel wizard tooltip
* Removed the comment from the test for the panel wizard

* Added a missing space between attributes in the pdf generator select
* Replaced the comment with an actual test for the export PDF section

* Fixed a `label` copy pasted too fast: the for attribute did not get updated.
* Removed the comment from the test for WYSIWYG

* Added the panel header text color to the ThemeSheet.xml
* Changed the name of the variable to fit other variables in the tab
* Updated color themes
* Fixed property name
* Fixed colortheme files and homogeneized property names
* Fixed codestyle
* Fixed consistency
* Fixed backward compatibility

  * Remove pages that are no longer used and where kept only for
    ensuring old URLs would still work

  * Use the proper macro for displaying user in email notifications

XWIKI-21791: The background of unread events displayed with "Load older notifications" is not grey anymore, but white
* Fixed a couple comments
* Added back the JS class addition removed in XWIKI-18880

XWIKI-21807: Error when adding two properties of the same name not properly displayed in class editor (#2803)

Fix broken test.

Remove the confusing caption style on the description.

This reverts commit b06b4c0e.

* remove unused import

* Remove mock() call not available in 14.10.x
* Use constants for user and document reference in the test

(cherry picked from commit f1401025)

* Introduce a new listener to cancel any save that would delete the document.
* Add a unit test.

Translations update from Weblate

Co-authored-by: trrenty <trrenty@gmail.com>

* Improved architecture of the headers
* Updated drag and resize handles to support keyboard interactions
* Use column titles as drag handles
* Added a keyboard alternative for resetting column sizes
* Fixed CSS organization
* Added a new translation key for non-sortable columns

Currently translated at 100.0% (4484 of 4484 strings)

Translation: XWiki Platform/XWiki Core Resources
Translate-URL: https://l10n.xwiki.org/projects/xwiki-platform/xwiki-core-resources/uk/

XWIKI-21784: Long names cause avatar to disappear from Drawer and the 'Close the drawer' button to be shifted out of view (#2783)

* Removed the max-width set on the avatar that allowed it to get squished down

* Added back style lost when removing the drawer dependency
* Removed some useless style
* Increased specificity for the padding-left of the submenu items so that they would be correctly offset

* Update webstandard test pages
* Updated the .pom with references to jira tickets for each page

Add missing Override annotations.

  * Use the right serializer for performing comparison: compactwiki only
    works if the wiki reference is set, here we need a local serializer
since we never want the wiki reference

  * Fail the data migration manager initialization if two migrations are
    found with same version.
  * Improve the check to actually check all migrations but only log
    warnings if the migration is ignored, to avoid blocking the
initialization.
  * Add a few more doc