Check whether email belongs to existing iNaturalist user

[albullington]

This will likely require work on the API, so may be blocked at the moment — whoever works on this should check to see how feasible this is on the backend before making UI changes. The signup screen should be split up, with a check to see if the email address already belongs to an existing iNaturalist user.

If it does, show a warning that the email address already belongs to an account.

If it doesn't, user can proceed to the rest of signup.

See Figma non-final designs here.