Cap the maximum value of mogrification parameters

[craigymunro]

In theory you could overload our asset servers by making several unreasonably large mogrification requests. For example:

http://images.eurogamer.net/2014/events/rezzed/IMG_2272.jpg/EG11/thumbnail/10000x10000

We should look into capping the mogrification parameters to a reasonable value. In practice I've rarely needed to resize past around 2,000 pixels, so maybe we should cap thumbnail and resize values to 2000x2000?