Split GitTabController into an Item, Container, and Controller

[smashwilson]

The first bit of #1436. Split the GitTabController into three pieces with distinct reponsibilities:

• GitTabItem implements the "item protocol".
• GitTabContainer fetches asynchronous properties from the Repository. It manages the git tab's loading state and default properties.
• GitTabController is left with only the action methods that are passed to the rest of the hierarchy.

This should make fetching mentionable users from GraphQL easier because we can do it within the GitTabContainer and not have yet another layer of "are these props here or not."