Enable basic branding

[craig-willis]

Problem

Branding items such as the dashboard instance name, header color, and logo are hardcoded. With https://github.com/whole-tale/girder_wholetale/pull/572, this PR adds branding configuration support via a new /wholetale/settings endpoint.

How to Test

• See https://github.com/whole-tale/girder_wholetale/pull/572 for test case