OmniAuthCallbacksController: Eliminiate @user

[islemaster]

Mentally tracking the state of this @user instance variable while working on this controller has been a small pain point that wasn't much work to eliminate, so here I've removed it and moved us one step away from side effects in these controller functions.

There are two key signature changes here:

void sign_in_user => void sign_in_user(User)
Instead of depending on instance state, this method now signs in whatever user was passed in.

void silent_takeover(User, AuthHash) => User silent_takeover(User, AuthHash)
This method used to sometimes set @user as a side-effect of the silent takeover, causing the takeover target user to be signed in, instead of the OAuth user passed in as the first argument. I've made a small change so that this function now returns one user or the other, and expects the caller to handle the result. Expected calling pattern is now:

user = silent_takeover user, auth_hash

[maddiedierker]

🎉 great clean-up!