Add realm level setting(s) for Open-specific values

[rhysyngsun]

What are the relevant tickets?

Part of #47

Description (What does it do?)

This adds a UITabProvider which adds an "MIT Open" tab to the realm admin interface as well as exposing the configured data via realm attributes (only 1 for now).

I also did some refactoring of the template provider to organize the properties under beans so they're not all at the top level.

Note: there are several other places that need to use this url too, I'm going to deal with that in a separate PR later. I only did enough here to prove that it works.

Screenshots (if appropriate):

How can this be tested?

• Go to the admin page and enter the home page url for your open instance (e.g. http://open.odl.local:8083/).
• Go to login via keycloak, the logo should now link to the url.

[rhysyngsun]

@collinpreston should be all set, although I'm not sure why you'd be hitting this error: https://github.com/mitodl/ol-keycloak/pull/52#discussion_r1562504826

[rhysyngsun]

Note you'll need to re-enter the setting, since the key changed.