Reduce code duplication by calling inputables from session.js

So the duplication between the new `inputables` and the existing `session.js` getter methods was driving me crazy: what if we fix a bug in one and forget to fix the corresponding bug in another? Something had to be done.

I present to you: All `sandbox.js` getters reduced to simply calling the corresponding `inputables`.

And believe it or not, I wrote a test extension that calls all these getters and logged the response so I could check they were behaving correctly ;-) We really need some kind of test suite...