Guide Favoriting!

This adds a menu item to guide view so users can favorite (and
unfavorite) guides. The icon is functionally set up to perform the
appropriate API call but it still needs some UI polish:

1. A different icon for the unfavorited state
1. Disabling the icon during API calls
1. Showing some sort of progress indicator
1. Tell the user what happened e.g. a toast with "Favorited"