Summary:
- Refactors account-sidebar internal package:
- Separates into smaller react components
- Makes DisclosureTriangle its own independent component
- Adds data to AccountSidebarStore to allow removal or addition of items for a
specific section of the sidebar
- Adds button and input and css styles to create categories
- Adds context menu to destroy a category
- Adds new method to CategoryStore to get the icon name for the categories of
the current account
- Removes some unused code
Test Plan: Manual
Reviewers: evan, bengotow
Reviewed By: bengotow
Differential Revision: https://phab.nylas.com/D2283
