25271 advanced installer

Complete refactoring of the installer to provide user-selectable "Install methods" (essentially install profile settings in Classes). Three methods packaged:

- Minimal (aGov without content type features)
- Standard (equiv. to aGov with no default content)
- Demo (equiv. to aGov with default content)

All three are tested and functional. All tests pass with the Demo option. 

To test:
- Install using the UI