Multiple credentials - NOT READY TO MERGE

First step towards https://github.com/LearningLocker/learninglocker/issues/121

### Summary

The code below 'works' in the sense that if you create a new database, Learning Locker still accepts statements as before. 

It's incomplete in the following ways:

* It doesn't yet add any new functionality, just changes the data structure
* It doesn't convert the existing data structure, so you need to either wipe your database or edit the structure by hand. 
* I haven't tested the document APIs (it looks like there's no special code for these though?)

### Questions

* Any comments on the code so far (esp. filters.php)? Is there a more laravelian way to do this rather than nested for each loops?

* Happy with this direction or prefer I try to do this as a plugin? 

* What I can do so that people can install this with their existing database?