Gerrit administrators can put per-project themes in
$site_path/themes/{project-name}; the same header/footer/CSS filenames
are supported as for sitewide themes. These are inherited and cached
per-project and exposed via GET /projects/X/config.
Themes must be provided by a site admin rather than arbitrary project
admins, so the site admins can be responsible for making sure themes
do not introduce XSS vulnerabilities.
Change-Id: I065d9e6d4df9275b963bb142ec11f66b5604678b
1e49e143ec