fix(Comment): replace richEditor mixin with NcRichText

[Antreesy]

• Resolves: get rid of richEditor mixin direct usage, so it can be removed from lib or reworked
• Based on the solution Talk uses

Summary

Before	After

TODO

• comments endpoint returns message as hi @alice @bob@nextcloud.com, NcRichText requires hi {mention-1} {mention-2} to be fed, as it can't parse some characters to mentions
• activity app endpoint returns message_rich, which might be used, but current solution is compatible with and without activity app
• markdown is supported by NcRichText, but not enabled to reduce changes

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[Antreesy]

/compile /

[Antreesy]

/backport to stable31