Consider moving starter site to separate project

[yaph]

Pros

• Changes to the starter site may be independent of core changes.
• Possibly better handling of multiple starter sites.

Cons

• Core changes that require starter sites changes require simultaneous releases.

Possible structure

- base/  # all files that are used on all sites
- starter/  # additional files and overrides
- theme1/  # additional files and overrides
...

[yaph]

A cookiecutter based approach could be interesting as well.