[web] Keep submit buttons always on the bottom of the ThreadSettingsModal

Summary:
Context: here

Improving user experience of ThreadSettingsModal

Main changes:

• keep submit buttons always on the bottom
• move error message above submit buttons

Test Plan:

1. Go to any chat
2. Go to Thread Menu > Settings (ThreadSettingsModal)
3. Switch between tabs

Now, all the submit buttons in form are keeping on the bottom of the modal.

Reviewers: tomek, atul

Reviewed By: tomek, atul

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5220