[web] Introduce barebones EditThreadAvatarMenu

Summary:
Similar to EditUserAvatarMenu, but for threads.

This diff only introduces the "Reset to default" functionality. Next diff will add image avatar support. The diff after that will add emoji support.

Test Plan:

1. Set thread avatar
2. Press "Reset to default" button and observe that thread avatar is unset in DB

Reviewers: ashoat, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8840