Fix broken multiple-domain project configurations

Project sites that contain multiple configurations (domain, TLS cert/key) are broken in GitLab Pages. Only the first TLS configuration is actually enabled, because in readProjectConfig, references to only the first JSON entry are passed to addDomain.

This MR fixes the issue, and also removes the for ... range copy that was perhaps not meant to be introduced by the original author.

I hope this is merged in quickly, because GitLab.com is currently serving the wrong SSL certificates for GitLab Pages that have separate certificates for the site.com and www.site.com domains.

This was originally reported on the support forum some days ago. (Marked as a confidential issue, so may be hidden from your view.)

mentioned in issue gitlab-com/support-forum#1740 (closed)

Thanks @jpap! The only thing this MR misses is a test - I'll take this up and try the fix merged soon.

I agree that it makes sense to backport this. Wdyt @ayufan? To 9.0.x or just 9.1?

mentioned in merge request !27 (merged)

Closed in favour of https://gitlab.com/gitlab-org/gitlab-pages/merge_requests/27 but all credit to @jpap

closed