[Feature Request] Tagging Messages

It would be amazing if we could mark messages for ourselves with multiple tags and have a built-in way to search for messages with specific tags. On servers there could be a permission for marking with tags.

Some examples:
- Someone sends in a suggestion or a bug report for your project, you could give it a TODO tag and a priority tag
- Students are messaging eachother with notes on subjects, they could mark messages with the subject's name (Maths, Physics, CompSci, etc.)

In the search bar, you could search for messages with tags with "tags:Maths,TODO" and it'll only display messages that have all the tags you listed.