Create controller that performs patron dump and serves barcode+username

[leonardr]

The backlog item #24 was recently confirmed to me as an important requirement. Once we implement it, the code Courteney wrote for #8 will stop working, because we will no longer have direct access to username and barcode.

We can still provide this information to the patron, but we need to get it from an ILS patron dump every single time it's requested. The performance effect (and the effect on the ILS) means that we shouldn't grab this information every time the patron syncs their loans. We need to put this code in a standalone controller which the client fetches only once, upon login.

We need to come up with a representation format (there are no books in this hypothetical document, so OPDS doesn't make sense) and a link relation so we can tell the client where to go to get detailed patron information.

[leonardr]

When designing the representation format keep in mind that #33 requires that it eventually include a link to a barcode image.

[leonardr]

Now that OPDS is out of the question, I think Authentication for OPDS would be a good format to copy.

https://docs.google.com/document/d/1-_0HHt664bDjybtCauBJXUSDXiT-Clg1sZUVNxHyLjw/edit#

So the patron dump might look something like this:

{
"username": "bellhooks",
"barcode": "23333908579384",
"links" : {
"barcode": "https://circulation.librarysimplified.org/barcode"
}
}

Maybe we could then get this included in the Authentication for OPDS spec.

Note that at this point I'm just saying "barcode" instead of "authorization identifier". The link really is pointing to a graphical barcode that can be scanned, and the link relation should make that clear.

[leonardr]

Set up the controller at /me and add a link to /loans that points to /me with a link relation of "http://librarysimplified.org/terms/rel/account". Use the format I give above to describe the patron. We'll change it later if necessary.