Adds a config option extensions.passmanager.strip_hostnames which takes
a comma separated list of hostnames to remove from the beginning. If a
corresponding directory is found, all entries are evaluated. The normal
hostname is also tried for backward compatibility. New entries are
stored to the shortend hostname.

fix updating passwords for HTTP authorization

* Save FirefoxAccounts to default JSON storage
* Add new config option save_as_username to allow saving passwords to files called after the corresponding (sanitized) username instead of passmanager