Update email regexes to detect +

[rebecca-shoptaw]

Closes #9325.

Fix. Adds + detection to email regex for live & backend validation.

Technical

Simply added a specification that one or more characters that are not a plus sign must appear before the @. Also made the regexes more consistent, preferring the current account.py regex which also confirms the existence of a period after the @.

Testing

1. Log out (if logged in)
2. Go to /account/create
3. Attempt to make an account using an email with a plus sign
4. A realtime error should appear
5. Ignore the error and try to submit
6. An HTML-generated error should appear and prevent submission

Screenshot

(pending testing on GitPod)

Stakeholders

@cdrini

[rebecca-shoptaw]

Turns out we can fix this on the IA side instead! Handing over to them to update the endpoint so it throws an "already registered" error if the + alias matches an existing account.