Improved support for external File content

Problems with Files in 2.0

• It is not currently easy to use a file in multiple places throughout Moodle and update them all at once
• It is not currently easy to create a simple shared "course repository" for teachers to use

Proposed solution summary

1. Extend Files API with a new "location" concept, with all files now having a "locationrepository" and a "locationreference", which defines where the content of the file is.
2. All files copied into Moodle have a "locationrepository" of "local", but others will specify a UUID (usually a URL with file-specific tokens in it, created by the original user who placed the file).
3. Improve the filesystem repository plugin to make it easier to create folder repositories on the fly
4. Add support to the filepicker UI to add "linking" to more repositories that support it, including the server files and filesystem browser.
5. Virtual files are served via pluginfile.php URLs just like normal files:
   1. pluginfile.php uses module callback to determine access, and if it passes then
   2. pluginfile.php calls a file logging subsystem to log the fact that this file is being served (useful for copyright reporting, for example)
   3. (virtual files only) pluginfile.php uses repository callback to get the content of the file, and streams it to the user with appropriate mimetypes etc

Note that the original URL of files in external repositories are never revealed to users.