We should offer the "mark as resolved" and "mark as unresolved" actions in the UI, to operate on a topic. Help Center page here:
https://zulip.com/help/resolve-a-topic

This will be part of the topic action sheet. At the moment we don't yet have a topic action sheet; this can be the first item in it, or this issue can come after #348 which will also add items to it. See #348 for where the topic action sheet should appear in the UI.

Related chat discussion:
https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/resolving.20topics/near/1829650

Prerequisite:

• Handle moved messages (update_message events, 2/2) #150