Summary:
This diff:
- Improves the styling of the tabs in the preferences sidebar.
- Adds an optional param to section cofnig that puts an "account" submenu beneath the tab item.
- Renames preferences "sections" => "tabs", and renames the PreferencesSectionStore to PreferencesUIStore. I think we should include "UI" in more of our stores, and I think "tabs" is a good idea because it's unambigious—there's no way you could confuse it for a "section" of the NylasEnv.config tree or think it deals with actually saving prefs.
Test Plan: Inspect visually
Reviewers: evan, juan
Reviewed By: evan
Differential Revision: https://phab.nylas.com/D2296
