Add pre-registration validation for IA email and username

[rebecca-shoptaw]

Closes #7080.

Fix. Adds checks for any existing Internet Archive account with the selected email or username before submission of registration form. This will notify users real time if the username they select is already taken, so that they can choose a new username rather than being auto-assigned one with random digits appended.

Technical

Because of the scope of #2055 , I've limited this PR to only adding real-time validation for the fields, not preventing submission with a taken username. This means that if a user elects to ignore the "username taken" error message and submits anyway, the account creation will go through and random digits will be appended to their chosen username.

Submission prevention for this and other failing backend checks (i.e. taken OL email address) will be added as a part of #9205.

Testing

1. Log out (if logged in)
2. Hit "Sign Up" or go to /account/create
3. Enter an email address for which an IA account but no OL account exists (i.e. [email protected])
4. An error message should appear (on tab out/de-select) with the warning "An Internet Archive account already exists with this email"
5. Enter a username for which an IA account but no OL account exists (i.e. rstesting)
6. An error message should appear (on tab out/de-select) with the warning "An Internet Archive account already exists with this username"

Screenshot

Stakeholders

@mekarpeles @seabelis @cdrini