Rework how duplicate entries in message_tags are handled

[miaulalala]

Summary

• Remove the unique index constraint from message_tags. The current implementation doesn't work for a few different reasons and has caused a need for some hacky solutions like Catch duplicate inserts of tags #4821.
• Add a cleanup job for duplicate entries
• Add a cleanup job for orphans.
• The join query in getAllTagsForMessages() needs to handle selecting unique values only.
• Additionally, to speed up the join query, a shortened index like seen in nextcloud/server#26070 could prove useful.