[web] Rename mostRecentReadThread to mostRecentlyReadThread

Summary:
In D3341 we realized that it might be a good idea to rename mostRecentReadThread to mostRecentlyReadThread. This diff contains this rename with the corresponding selector rename to keep things consistent.
Depends on D3386

Test Plan: Flow

Reviewers: benschac, atul, def-au1t

Reviewed By: benschac, atul

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3387