- 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.
- This ensures content notifications are not translated to receiver
language.
- This adds actual plaintext support for content notifications (Was
previously just HTML as text view).
- Shares same base class across all mail notifications.
- Also cleaned up existing notification classes.
Future cleanup requested via #4501
