Add ability to search by user, mentions and attachments.

[zDEFz]

I type DEF, I expect that the respective user ID is recommended and can be used. Discord does this through the "mentions:" filter.

[insertish]

Alternative title: decide whether to use elasticsearch, meilisearch, or anything else then implement accordingly.

[tadghh]

Alternative title: decide whether to use elasticsearch, meilisearch, or anything else then implement accordingly.

Meilisearch for the "pure rust" marketing? I heard elasticsearch was bought out and changed their license too.