Prevent credentials from being filled in without validation

[aeneasr]

Assuming we have strategy "password" and "google-oidc" enabled. Assuming foo@bar.com has a registered account at hive using the password strategy. If another user with "google-oidc" signs up, and also has foo@bar.com as email, this should not be allowed. This should work with all strategies.

Instead, the user should be requested to "combine" his/her accounts by linking them.

We need to make sure that our validator checks for this usecase and rejects any requests not compliant with this policy.

• Disallow oidc strategy sign up for identities that already have the email registered (form error)
• Disallow password strategy sign up for identities that already have the email registered (form error)
• Write test for oidc strategy
  • Test double sign up with existing identifier from password strategy
  • Test double sign up with existing identifier from oidc strategy
• Write test for password strategy
  • Test double sign up with existing identifier from password strategy
  • Test double sign up with existing identifier from oidc strategy