Fixes #4919: Saving a model before Relationship fields have fully loaded causes data loss

[n-devr]

Description of changes

Enhances the 'Save' button on the admin UI to act as a loading indicator while Relationship fields are populated and prevents the user from saving the model until all Relationship fields have completed loading.

Related issues (if any)

4919

Testing

• [x] List browser version(s) any admin UI changes were tested in:
  • Chrome Version 79.0.3945.117
• [x] Please confirm you've added (or verified) test coverage for this change.
• [x] Please confirm npm run test-all ran successfully. Tests passed at the same rate as they did prior to changes. There are many linting issues (in unchanged files) and some tests that remain in the 'Pending' state indefinitely.

[n-devr]

Thanks @autoboxer - happy to help! Please let me know if there's any additional steps I should take to help get this merged.