[web] [feat] thread menu button hoverable

Summary:
remove menu button for threads

updated with hover:

note: arrow positioning updated in future diffs.

Test Plan: should render, look like screen shot

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3099