Retire encrypted flag in backups

[mymindstorm]

Emmm, we retired encrypted flag and have a new field to handle encrypted secret. This makes sense. I think we can do a migration when import backups, and remove all logic about encrypted flag.

Originally posted by @Sneezry in https://github.com/Authenticator-Extension/Authenticator/issues/485#issuecomment-632159860

[Sneezry]

If we remove the encrypted flag, the new backup will not be recginzed correctly in old version extension. That's okay, but I think it should be good to add a version feild into backups. And if a new version backup can be imported by old version extension, we can change the minor version, otherwise, change the major version. How do you think?

[mymindstorm]

@Sneezry We really only need to have one major version on backups and reject the backup if major version is higher than what we can handle. We can just add encSecret as an extra parameter and ignore it. Then, when enough users have a version that can handle the new format we can start removing encrypted from backups.

[Sneezry]

Sounds good