This introduces a new "controlled link" file type where the file is not
stored in Moodle - but Moodle will control the access permissions on the file.

Plugins can "freeze" a filearea which means Moodle will take ownership of all the remote
files of this type.

When accessing a file, if the "filebrowser" infomation indicates the current user can write to the file, they
will be granted temporary write access.

Part of MDL-58220