Support cgit alias sites

This allows creation of new top-level sites and aliasing of
existing repos into those sites (under arbitrary names).

For example, this will let us list only the zuul projects under
git.zuul-ci.org.  This scheme does not extend to the git protocol,
which we will start to deprecate.

Change-Id: I4c759f02d8d7e77439984d8ad1f012bd381fae59
Story: 2001382
Task: 6092
1 file changed