Edit login user functions to return the `User`

[michplunkett]

Fixes issue

https://github.com/lucyparsons/OpenOversight/issues/1010

Description of Changes

Edit the login_user functions so that they return the user (removing the need for an extra database query) and create constants for the login values.

Tests and linting

• [x] This branch is up-to-date with the develop branch.
• [x] pytest passes on my local development environment.
• [x] pre-commit passes on my local development environment.