Reject duplicate resources with useful feedback

This is a:

[x] ✨ New feature - new behaviour has been implemented
[ ] 🐛 Bug fix - existing behaviour has been made to behave
[ ] ♻️ Refactor - the behaviour has not changed, just the implementation
[ ] ✅ Test backfill - tests for existing behaviour were added but the behaviour itself hasn't changed
[ ] ⚙️ Chore - maintenance task, behaviour and implementation haven't changed

Purpose - show useful feedback to the user when creation of their suggested resource fails

How to check -
- Log in as any user
- Go to the suggestions page
- Suggest a resource
  - A success message should appear and the form inputs should be cleared as previously:
- Suggest another resource with the same name and/or URL
  - A message should appear telling the user the resource wasn't created, and the form input should not be cleared:
- If the request fails 401, the user should still be logged out and sent back to the home page
- If the request fails for any other reason, a more general error message is shown.

[ ] ~I have written a title that reflects the relevant ticket~
[x] I have written a description that says what the PR does and how to validate it
[ ] ~I have linked to the project board ticket (and any related PRs/issues) in the Links section~
[ ] ~I have added a link to this PR to the ticket~
[x] I have made the PR to main from a branch named <category>/<name>, e.g. feature/edit-spaceships or bugfix/restore-oxygen
[x] I have manually tested that the app still works correctly
[ ] ~I have requested reviewers here and in my team chat channel~
[ ] ~I have spoken with my PM or TL about any parts of this task that may have become out-of-scope, or any additional improvements that I now realise may benefit my project~
[x] I have added tests, or ~new tests were not required~
[x] I have updated any documentation (e.g. diagrams, schemas), or ~documentation updates were not required~

CodeYourFuture / tech-products-demo