- 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.
Ensures some sensible text is always in webhook text data.
Also aligned some notification reporting to use centralised activity
system instead of custom success events.
For #4216
