[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