Add support for signatures and previews

[MaxJohansen]

Signatures

Templates that take a signature (currently the employee template is the only one) can now accept an optional query parameter indicating which signature they wish to use. The signature is indicated by its name (e.g. gavinbelson) and will be scaled and drawn onto the diploma in the position indicated in templates.json.

To see which signatures are available you can use the /signatures.json endpoint.

Example usage:
/employee?name=Joe&date=today&boss=Donald%20Trump&signature=donaldtrump -> A diploma for Joe signed by the worlds most famous neo-nazi fascist scumbag.
/employee?name=Jane&date=01.01.1970&boss=Donald%20Knuth -> A diploma for Jane with that needs to be signed by Donald Knuth IRL

Previews

To preview templates or images, you can use the new /preview/<name> endpoint, where indicates either a template name or a signature name. Template previews have boxes and text indicating where the field values will go. This endpoint also takes the optional query parameters height and width to produce a "thumbnailed" version of the preview image which is nice if you only intend to render them in a list view on a tiny device anyway.

Example usage:
/preview/template/employee -> A full-sized version of the employee template decorated with boxes that indicate where the fields are.
/preview/template/employee?width=300 -> Same as above, but sized to fit into 300 pixels horizontally.
/preview/signature/waltdisney?width=200&height=80 -> Walt Disney's signature (black text on white background) resized to fit into a 200px by 80px rectangle.

JSON structure

All of the JSON served is now in its most minimal form: the location of the fields is not (currently) relevant to the front end applications using this web API, so that information is stripped out. An example using the Employee template:

{
   "Employee" : {
      "signature" : true,
      "fields" : [
         {
            "name" : "Name"
         },
         {
            "name" : "Date"
         },
         {
            "name" : "Boss"
         }
      ]
   },
  [... more stuff here...]
}

and the Gavin Belson signature:

{
  {
    "id": "gavinbelson",
    "name": "Gavin Belson"
  },
  [... more stuff here...]
}

[IsakSundeSingh]

These dirty hacks aren't DRY

[IsakSundeSingh]

Isn't it better if we have the preview paths for images and signatures separated?
E.g. Preview a template: /preview/template/<name> and preview a signature: /preview/signature/<name>.
This will make it more future-proof and we can more easily add support for previewing other things as well, without breaking changes to the API.
It also reduces the chance of a name conflict.

[IsakSundeSingh]

Remove these descriptions. They aren't even necessary.