ikiwiki hosting internals/ todo/ anon git access by external domain

Currently, git://externl.example.com/ doesn't work. For this to work, ikisite needs to manage links in /var/lib/ikiwiki-hosting-web/git/ for each configured external domain. --Joey

> done .. note that the Branchable tab still shows the internal domain in git urls. It's generally ok to use, unless you have some existing domain that is moving to Branchable, etc. --Joey