Add a comment filtering mode

The creator of a room should be able to decide for a comment filtering mode to restrict the comments available in the comment stream so that comments can be filtered before showing them in the comment list view. I'd suggest two options:

• Blacklist: All comments are visible but moderators and the lecturer can blacklist a comment that makes it hidden
• Whitelist: Only in the comment moderating view every comment should be visible and moderators (and the lecturer) can mark comments as visible to be shown in the normal comment view

The comment moderating view

Moderators should have access to a view that shows every comment disregarding their visibility.
This view should focus on the following:

• showing the stream of comments, newest first
• depending on the mode: either make a comment visible or hide it
• having an option to view the comments that - depending on the mode - either got hidden (for the blacklist mode) or made visible (for whitelist)

Implementation:

• it may be useful to have a "visible" flag for the comment with three possible values: "hidden by moderator", "visible to audience", "not yet moderated".

Notes:

• The whitelist mode needs to either straight up disallow editing a comment or have an update on a comment put it back to not being visible and needing validation
• moderating view will need a "decide comment by comment" view so that spammy rooms can also be managed