[web] Push <form> tag down from ThreadSettingsModal to child "Tab" components

Summary: Addresses feedback from @palys-swm in a previous stack. Makes ThreadSettingsModal cleaner.

Test Plan:
Things continue to look/work as expected:

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3928