FS-1530 Suppress federated errors when removing users from conversation

[lepsa]

Checklist

• [:heavy_check_mark:] Add a new entry in an appropriate subdirectory of changelog.d
• [:heavy_check_mark:] Read and follow the PR guidelines

Changes

Make the following routes fault tolerant of federation members being unavailable, and adding tests to cover this situation. The two routes removing members from conversations are covered by the same test, as they quickly converge on the same code path.

• DELETE /conversations/{cnv_domain}/{cnv}/members/{usr_domain}/{usr}
• DELETE /conversations/{cnv}/members/{usr}
• DELETE /teams/{tid}/conversations/{cid}

[elland]

This seem to have somehow changed the result of the "POST /federation/search-users : Found (fulltext)" test to return a lot more users than the test expected. I couldn't reproduce this issue locally yet. Investigating further.

[lepsa]

Looks like CI is stuck, and I haven't got authorization to merge into the repo.