Add routing classes

Adds a Route class hierarchy for various URL schemes and a Router
class to convert Route classes to and from URLs.

Right now we only have two(-ish) schemes, but in the future we want
to be able to show related images which are not present on the current
page and need shareable URLs for those as well; also we might want
to specify other things in the URL than the current image (the reuse
box being open was one thing discussed); this will be a good framework
to add features like that.

The MainFileRoute class will be used by #416.

Change-Id: I489126a0ada37f91a22a2f48a4e686140a28d162
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/371
Merge "Add routing classes"