[web] Move padding from modal_body to tab_body in ThreadSettingsModal

Summary:
We want each of the ThreadSettingsBlahTab components to have some padding so their contents aren't right against the edge of the surrounding Modal. However, we do want the Tabs to extend all the way to the edges and be closer to the Modal header.

We can't put these divs inside <Tabs.Container> because it expects its children to be typed a certain way. As a result we have to copy/paste the <div className={css.tab_body}> tag for every tab. We could move this padding to the children Tab components themselves, but setting the padding in ThreadSettingsModal seems like the right layer of abstraction to me.

Depends on D3929

Test Plan:
Before:

After:

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3930