Load branding preferences from API endpoint

The instance API response now includes the "config" dictionary as well
as the the custom logo url. I have added code to set these values on the
environment and raise a notification that the environment changes. The
various UI widgets respond to the notification and change their colors
accordingly.

I removed navBarTintColor from the environment and replaced it with
primaryColor (and secondary color) to avoid confusion. The color may be
used for more than just nav bar tinting.
Merge pull request #66 from jwalgran/topic/branding

Load branding preferences from API endpoint