Change default oAuth Host and allow config

[shaunanoordin]

PR Overview

This PR changes PJC's oAuth host:

• The default oAuth host is now www.zooniverse.org instead of panoptes.zooniverse.org, to reflect current changes to the backend. That's right! oAuth users now sign in via our "main domain" instead of the panoptes. sub-domain.
• The oAuth host can now also be changed either by setting ?oauth-host=... in the browser, or setting OAUTH_HOST in the env build variables. This brings the oAuth host config setting in line with other editable items in the PJC's config.

To Custom Front End developers:

• This change will allow single sign on across all *.zooniverse.org domains (if you use the oauth.js code)
• If you implement these changes, you should see no changes to sign-in functionality. The only difference should be that when users login, they see a different Zooniverse.org domain asking for their user/pass.
• Users who are still logged in when you push the changes, however, might find themselves logged out afterwards.

Tested using a local copy of PJC and localhost Education API Front End, which was used to verify the single sign on.

Status

Ready for review. If all goes well, this might only require a minor version bump since it's not breaking any existing functionality - but let me know if you think otherwise.

[camallen]

LGTM

I think this can be a patch version bump. I can't think of any problems for CFEs auto upgrading to this version of the library. I think most users of this code will want these changes

[srallen]

For semantic versioning, though, I think minor version bump is more appropriate since this isn't a bug fix, but introducing an API change to allow single sign on.

[srallen]

@shaunanoordin are you ok with merge, versioning, and publishing of this?

[shaunanoordin]

👍 Can do! I'll get this merged and then bump the version. Uh, let me pull up my notes on how to public PJC though.