Option to disable username and password login (BB-13647)

Mentioned by Brian Westrich in issue 7016:

If there was a checkbox I could click in my bitbucket profile settings that disabled all logins except those via my google account or ssh keypairs, I think I'd have all I need.

The lack of 2FA has stop many users from start or continue using BitBucket. While waiting for Atlassian ID implementation, disable email/password login can let users benefit from 2FA protection right away.