[web] Fix ThreadSettingsModal and flip switch to enable EditThreadAvatar

Summary: Merged the fixing of layout and "flipping of switch" for convenience.

Test Plan:
Looks as expected (and works as expected). Here's how each tab of ThreadSettingsModal looks now:

Reviewers: ashoat, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8919