Rewrite Comment API

The current Comment API is quite old and some important features are missing. We (TU Berlin) would like to implement a new Comment API for Moodle 4.0.

We are thinking of the following (new) features:

• Plugins have to define their comment areas in db/comments.php.
• Two possible comment structures (plugin defines which one to use): flat (all replies are below the other as it is currently) or question/answers (replies are indented and associated with a parent comment).
• Possibility to comment under a pseudonym. A plugin defines whether pseudonyms are allowed or if the user is even forced to comment under a pseudonym (e.g. mod_assign could force pseudonyms when the grader identity is hidden from students, cf. MDL-66987).
• A plugin has to implement a class that represents one comment section (i.e. a context, component, comment area, item id) and that controls the permissions and enabled features.
• Notifications: Users can (un)subscribe to a comment section, to a comment (getting notified when there are replies to a certain comment), to all comment sections (all itemids) within a (context, component, area). And maybe even (un)subscribe to all comment sections of a course?
• Users can decide if they want to be notified immediately or in a daily digest.
• Plugin optionally defines which users should be notified by default (e.g. mod_assign could notify the grader by default when a student comments the grade).
• Users can view the newest comments in all comment sections (all itemids) within a (context, component, area). Maybe even the newest comments in (all contexts in a course, component, area)?
• Permalink to a comment and link to the page that the comments (within an area and itemid) belong to.
• Allow users to edit their comments within $CFG->maxeditingtime.
• Comments can be upvoted with a thumbs up (in order to avoid simple comments like "+1") and a thumbs up counter is displayed. Users can undo their upvote. Plugins may disable votes.
• Possibility for a plugin to add custom/meta data (JSON) to a comment (for example, a video plugin could add the point in time in the video that the comment refers to).
• In addition to the existing capabilities (moodle/comment:delete, moodle/comment:post and moodle/comment:view) we want to add: moodle/comment:editany and moodle/comment:postpseudonym

The old API should be deprecated and removed in Moodle 4.4.