sent password reset email even for getpersonas migrated accounts (bug 998477)

Fix [bug 998477](https://bugzilla.mozilla.org/show_bug.cgi?id=998477)

At the moment, even if we can't reset a user's password (because her password
is empty), we still display a message stating that an email was sent to
reset it. This is now fixed.

Also, we have our own way of checking passwords, so the ```AbstractBaseUser.has_usable_password``` checks aren't loose enough for us (for example it checks the ```settings.PASSWORD_HASHERS``` which we don't necessarily use). An exemple is for passwords for migrated getpersonas accounts which are b64encoded. We thus monkeypatch this ```has_usable_password```, waiting for PR #63 to be merged.

@washort please check the ```# TODO``` I added in this PR if yours (#63) is merged after this one.