Don't prepopulate password after a login is blocked

[rfk]

Migrated from mozilla/fxa-auth-server#2605

If a user is blocked because they had a bad password, they need to submit their password again after they use an unblock code.

We, however, prepopulate the bad password in the login field making it too easy for users to submit the bad password again.

This problem was initially documented here:
mozilla-lockwise/lockwise-ios#662

and then https://github.com/mozilla/fxa-auth-server/issues/1998

┆Issue is synchronized with this Jira Task
┆Issue Number: FXA-740

[vladikoff]

from mtg: remove the autofill after you used the unblock code