Simplify/Split auth_controller

[nelsonic]

As noted by @SimonLab in the PR https://github.com/dwyl/auth/pull/159 the auth_controller is way too big. The file is currently 507 lines. This is far too long.

https://github.com/dwyl/auth/blob/76c0161d038e3d8c3840097c37e03c1d967f6b67/lib/auth_web/controllers/auth_controller.ex#L497-L507

Todo

• [ ] Review the file and determine if we can extract generic/helper functions
• [ ] Extract email+password related auth code into its own file, filename at your discretion.
• [ ] Remove OAuth (Google / GitHub) related code

[nelsonic]

Gonna do this as part of #207 👌