Added 4 more Plinth tasks, completed 3.

The includeSubDomains issue is still a problem, but that'll probably
take a lot of work over time.
Added new TODOs to Plinth and Infrastructure:

- Make a module manager (1.0)
- Create an etckeeper module (1.0)
- Separate modules like Apache (2.0)
- Support SSO (2.0)
- Split Plinth into multiple Debian packages (3.0)