Allow overriding of base URL from project

$CI_PAGES_URL will be set to the default Pages host, ignoring custom
domains. This means that we get
https://mesa.pages.freedesktop.org/mesa3d.org/ for the base URL when
running in the main repository, which is not what we want.

Allow this to be overridden by a project setting to define the best
canonical URL.

Signed-off-by: Daniel Stone <daniels@collabora.com>
2 jobs for master in 15 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #3092558
pages

00:00:15

 
  Deploy
passed pages:deploy

00:00:01