Add $uid and $password templates to 'files_external' configuration

Pull 'uid' and 'password' variables from PHP session 'smb-credentials'
array - if existing - and allow them to be used as $uid and $password
templates when configuring system mounts (in data/mount.json).

This allows to dynamically mount users' home directories using their
login credentials and comes handy when using the 'user_ldap' authentication
App/backend.