Multi-step installer/wizard #103

We should improve the way SE is installed by giving it a clear wizard for progress and checks.

Steps:

1. Validate requirements (should be able to work even without Symfony installed through composer)
   • See Installer checklist #94
   • Handled in https://github.com/SyncEngine/installer
2. Validate structure (either extracted by installer or manually deployed)
3. Install database (doctrine)
4. (skippable) Configure process manager (messenger)
5. (skippable) Configure mail sender
6. Register admin user