Accessing GitHub libraries and the NRW database

[EJMFarrow]

Needs to be merged after metadata PR.

• Allow access to GitHub question libraries from STACK library page.
• Allowed libraries are set on the STACK settings page via a JSON object.
• Cache of STACK directory structure and question renders for a GitHub library can be reset by any user with access to the library page. (They're much more likely to be changed than local libraries.)
• Involves much re-writing of the directory structure creation code in questionlibrary.class.php to make it more generic. (We'll need to carefully check the normal library has not be broken in any way.)
• Allow search of NRW database, display of some retrieved metadata and import of questions.
• Allow upload of questions to NRW - page available from STACK dashboard. Returns NRW question id and whether the question is a duplicate.
• Separate capabilities for accessing external libraries and for uploading questions.