Swapped back handling to instead be pre-determined instead of being
based upon session/referrer which would cause inconsistent results when
referrer data was not available (redirect to app-loaded images/files).
To support, this adds a mechansism to provide a URL through request
data.
Also cleaned up some imports in code while making changes.
Closes#4656.
- Moves guest user caching from User class to app container for
simplicity.
- Updates test to use simpler $this->users->guest() method for
consistency.
- Streamlined helpers to avoid function overlap for simplicity.
- Extracted user profile dropdown while doing changes.
