Enable gitlab pages

Fixes #228 (closed) and supersedes !163 (closed). Enables easy resolving of #61 (closed).

The test page is online at https://test.archlinux.org/. Its project is here: https://gitlab.archlinux.org/svenstaro/pages-test/pages It uses the GitLab built-in Let's Encrypt integration.

Changes are already rolled out.

Upon inspection and merging, the reviewer must:

• Remove hetznerdns_record.gitlab_pages_test_* from archlinux.tf and run terraform apply
• Remove the project https://gitlab.archlinux.org/svenstaro/pages-test

.gitlab/issue_templates/New GitLab Pages Project.md

+<!--
+This template should be used by DevOps members when adding a GitLab Pages project to GitLab.
+
+In order to use GitLab Pages with Arch Linux, you'll have to specifically request a custom subdomain
+below `pkgbuild.com` or `archlinux.org` to be assigned. We don't allow random projects to use Pages
+because of legal and safety reasons (we don't want people to be able to trick others into thinking
+something hosted below one of our domains is official).
+-->
+
+# Procedure for adding a GitLab Pages project to GitLab
+
+## Details
+- **Project name**: hello
+- **Desired subdomain**: hello.archlinux.org
+
+## New Pages site checklist
+
+1. [ ] Have a pipeline that outputs some static HTML to `public/` during the build.
+1. [ ] Specify this path (`public/`) as an artifact path.
+1. [ ] GitLab should now recognize that you're trying to use Pages and will show some relevant
+       information at https://gitlab.archlinux.org/your-namespace/your-project/pages
+1. [ ] At this page, you'll also need to add your custom domain. Add the custom domain you requested earlier.
+       GitLab will then show domain verification information which you'll need in the next step.
+1. [ ] At this point, we'll need to add some stuff to `archlinux.tf`. It should look something like this.
+       Make sure to substitute the `your_domain` and `your-domain` strings accordingly:
+
+        resource "hetznerdns_record" "gitlab_pages_your_domain_a" {
+          zone_id = hetznerdns_zone.archlinux.id
+          name    = "your-domain"
+          value   = hcloud_floating_ip.gitlab_pages.ip_address
+          type    = "A"
+        }
+
+        resource "hetznerdns_record" "gitlab_pages_your_domain_aaaa" {
+          zone_id = hetznerdns_zone.archlinux.id
+          name    = "your-domain"
+          value   = var.gitlab_pages_ipv6
+          type    = "AAAA"
+        }
+
+        resource "hetznerdns_record" "gitlab_pages_your_domain_verification" {
+          zone_id = hetznerdns_zone.archlinux.id
+          name    = "_gitlab-pages-verification-code.your-domain"
+          value   = "gitlab-pages-verification-code=your-code-shown-by-gitlab"
+          type    = "TXT"
+        }
+
+1. [ ] Run `terraform apply` and go back to GitLab. Hit `Verify` and it should pick up the new domain
+       verification code. It should then also automatically begin fetching a certificate via Let's
+       Encrypt. That should take roughly 10min.