Fix 500 error while navigating to the `pages_domains` 'show' page.

Summary

1. The path of the "pages_domains" show page is of the form 'group/project/pages/domains/<domain_name>'
2. Rails looks at params[:id] (which should be the domain name), and finds the relevant model record.
3. Given a domain like foo.bar, Rails sets params[:id] to foo (should be foo.bar), and sets params[:format] to bar
4. This commit fixes the issue by adding a route constraint, so that params[:id] is set to the entire foo.bar domain name.

Tasks

• [#1447 (closed)/!993 (merged)] Error 500 when showing details for a Pages custom domain
  • Investigate
    • Get GitLab CI working locally
    • Reproduce the issue
    • Check if related to !1448 (closed)
    • Why did this happen now?
  • Implementation
  • Tests
    • Added
    • Passing
  • Meta
    • CHANGELOG entry created
    • Documentation created/updated
    • API support added
    • Branch has no merge conflicts with master
    • Squashed related commits together
    • Check for clean merge with EE
    • Added screenshots
  • Review
    • Endboss
  • Wait for merge

---

• Closes #1447 (closed)
• Closes #1448 (closed)