Handle Geo node URL parse errors intelligently

What does this MR do?

Stores the geo node URL as a single field in the database rather than as schema, host, port, and relative_url_root

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Since we no longer have to parse the inputted URL, we can raise errors neatly at validation time, turning 500 pages into rendered errors on the page.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #4158 (closed)