Simplify .gitignore

The only thing we want to ignore in src/ are themes. For example, a future src/adhocracy/adhocracy-client directory should not be ignored.
Additionally, including lib is not helpful, as it matches src/adhocracy/lib, which we obviously don't want.
[welcome] Redirect using relative URLs

This fixes some weirdness when the redirect would switch over to the configured domain (say adhocracy.lan) otherwise.