Avoid Sentry exceptions when attackers try to stuff invalid emails. The
browser performs form field validation that should normally prevent this
from occurring, so we just return 422 without validation error messages.
Also:
- extract redirect_to_session_magic_link helper
- some controller refactoring and cleanup
using the "standard" email regexp URI::MailTo::EMAIL_REGEXP. The form
field will validate this in the browser, but if bots are creating
identities, they can put whatever they want in here. So let's add some
protection against that.
The HtmlHelper regex was renamed here to avoid confusing Brakeman,
which does imprecise constant lookup and was confusing the two
constants, one of which uses `\A` and `\z` and the other does
not (intentionally).
ref: https://app.fizzy.do/5986089/cards/3276
Mike Dalessio