Enable per repo config

While plain gitiles allows per-repo configuration
(only template.customHeader), this was not available
via the gerrit config. The getConfig method always returned
an empty config. Return a per-repo gitiles.config instead, with
inheriting from parent repos.
For the global pages, the config from 'All-Projects' is reused.

Change-Id: I9fe15e09567e1a5119de7c251cdefc276f5a1c71