Handle moved messages in recent-senders data

[gnprice]

This is like #901 but for the RecentSenders data structure.

The effect on the unreads data, #901, was quite visible as a user, so that was an M5a launch-blocker issue. The same bug as applied to recent-senders data is a lot less visible: the main effect is that if a topic gets moved or resolved and you try to write a message there and @-mention one of the participants, we'll fail to adequately prioritize them in the autocomplete results. This is therefore a post-launch issue.

@PIG208 has an existing PR for this, which we can start from when we pick this issue up:

• #1418

References

• Existing PR for this issue (paused when we moved this bug to post-launch): #1418
• Previous issue of a similar flavor: #901