* add missing javadoc

XWIKI-12144: List properties with a lot of large values selected may get corrupted in the cache, and can lead to data corruption
* Fixed the wrong test for indexing to make sure that we never send a Collection value to Solr for indexing, since Solr seems to alter it

- Fixed test

Code style and bulletproofing:
- prevent issue when Message-ID change during MimeMessage#writeTo() at serialization time
- prevent null Message-ID to used in statuses by forcing MimeMessage#saveChanges() when needed
- helper for MailListener implementation
- fix a raise condition in integration tests

- Improve MailStatusResult to easily filter all error states
- Improve MailStatusResultSerializer to report all errors

- Defined a separate XWiki.WatchListRealtimeMessage document to define the template of the realtime WatchList messages
-- Document updates that change a single XWikiComments object are treated specially, displaying notifications about comments in a more user-friendly way
-- Some other cases are also treated specially to improve the user experience regarding the notification message
-- Right now the location of this template is hardcoded for the realtime notification method and the handling is not extensible. Should be improved for the future.
--- Also, no internationalization support yet, since the realtime feature is still in an experimental stage

XWIKI-12068, XWIKI-12128, XWIKI-12129, XWIKI-12130: Refactor the mail API for a better management of error reporting

XWIKI-12128: Use standard Message ID to track mails in place of custom X-MailID header
XWIKI-12129: Remove the usage of custom header X-BatchID
XWIKI-12130: When prepare phase fails, process waiting for completion hands forever
XWIKI-12068: Add reporting for mails that fail to be prepared
XWIKI-12140: All messages generated by the MessageMimeMessageFactory has the same Message-ID as the source

XWIKI-12139: Extension automatically registered from XAR import does not generate ExtensionInstalledEvent

* Disable the install button if no flavor is selected.

* Typo! Thanks tmortagne for having noticed it.

* Cleaning

* merge with default 4.10.4 configuration

* missing various since

* fix bug

[Misc] "null" doesn't exist in Velocity, use a non-existing $NULL variable instead to represent null