Handle auth in front end

[dpordomingo]

Fix #99

major changes:

• new route on the front side: /auth:
  • it will receive the state and code from GitHub OAuth callback,
  • it will call through AJAX the (rename) backend /api/auth endpoint, and handle any possible error,
• UI_DOMAIN is no longer needed in Helm charts

benefits:

• the backend /api/auth endpoint will be standard in terms of our handler.RequestProcessFunc,
• the backend will communicate with the front end as the rest of the app:
  FE → BE → response + status

possible improvements:

• I realized that /auth route is not really-really needed because the auth thing could be handled by / itself if code is received.

[smacker]

1. You can't read POST payload in javascript

So your idea FE->BE->reponse doesn't work.

2. You don't handle errors from provider:
   https://tools.ietf.org/html/rfc6749#section-4.1.2.1

And we have to provide those errors to the user because those are problems between user&provider, not our app.

[dpordomingo]

• There is no post payload to read,
• errors during auth phase are returned to the FE as usual.

[dpordomingo]

It must be rebased over master and refactored considering #144

[dpordomingo]

PR rebased over current master.

This PR should not be merged till it can be coordinated with #201

PR now includes changes proposed by @smacker at #150:

• 35d2f5b refactoring
• 0e95b7a Remove redundant UI_DOMAIN
• a3f08a7 Fix new test from master

It was also included some minors:

• 52b7f45 Log OAuth errors errors

[smacker]

Oh. You cherry-picked my commits. Okay.

One small comment about error handling.

[smacker]

if you return (string, error), it makes sense to handle error here too.

[dpordomingo]

yesssss, you're totally right; thanks for spotting it!

[dpordomingo]

Ready for a second pass @bzz @smacker

• 3f6e7f5 error handling
• 737ce23 use /oauth-callback as requested by Update GitHub OAuth callback when Auth PR is merged #201