[web] Tweak layout of ThreadSettingsGeneralTab

Summary:
Make some tweaks to ThreadSettingsGeneralTab to space out the form elements a bit and in a more consistent way.

Depends on D3932

Test Plan:
Here's how it looks before and after:

Before:

After:

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3933