Support filtering by IDs in search, even if the user is not in client cache

[Jackenmen]

Problem

I want to search for messages from a user who got banned from (or left) the server. Since the search suggestions only show (online) server members, that's not possible.

Proposed solution

I think the client should just support providing an ID to filter by. It's not like the API call doesn't just operate on IDs anyway. This applies to user: and mentions: filters.

Notes (optional)

I looked for issues and couldn't find a duplicate, though I should note that it's hard to search for "search" when every issue/discussion has an "I searched for existing discussions ..." checkbox.

Checks

• I searched for existing discussions and didn't find a duplicate.

[hampus-fluxer]

Implemented in #1663 – will soon be deployed on the canary client. https://fluxer.app/download to get the latest client.