Centralize login information

[patrickfeltes]

Right now, the login flow happens over SplashScreenActivity and LoginActivity. It would probably be better to figure out how to have some sort of central login flow where it is easier to understand what happens when (JWT refreshing, etc), and also allows for unit testing.

This also doesn't currently handle the case of a JWT expiring while the app is in the background (making all requests afterwards fail). Even though this would be a rare occurrence (and could be fixed via force quitting the app), we should still try to handle it more gracefully.

This will definitely require some thought, so it is best that we try to do this after the event is over.

[patrickfeltes]

My initial thought for JWT refreshing was to add an interceptor that first parses the JWT to see if it's expired and then runs refresh before the call. Based on this: https://stackoverflow.com/questions/22450036/refreshing-oauth-token-using-retrofit-without-modifying-all-calls
we should not use Interceptor for this. Should investigate more about why