storage: Component to select a directory

A direcory selection and validation components could be useful for other modules like Deluge (#949 (closed)), Transmission, MLDonkey, Syncthing, Web sharing, minidlna. Related issues #950, #1679 (closed), #1681 (closed).

Consider following features:

• allow to set a default path
• allow users to specify a custom path
• show a hint/link to the user to enable samba sharing if not already enabled
• directory selection contains shared paths if sharing is enabled
• check that a directory exists and is readable by the user (service)
• optionally check that a directory is writable by the user (service). A variable 'reserved_usernames' could be useful here. Some modules may not need write permissions (minidlna, web sharing).
• add service user to the freedombox-share group if write permissions are required.