Add media viewer [alpha]

* Create a special overlay to maximize the viewing area for images.
* Hijack links to image file pages.
* Make the uploads dashboard HTML more similar to thumbnails in articles
  so that it works there as well.
* Add Router#checkRoute so that the current route can be checked again
  after something is loaded dynamically.
* Make mf-toggle.js resistant to all kinds of hashes.

To test click on a thumbnail in an article (e.g. a thumbnail added to
a page by a mobile upload). Then you can tap anywhere on the image to
see the description and Details button. Try also rotating your phone
while the overlay is open.

Change-Id: I38bcbdf931ae48356a4db2bb9785d639cfbf8e4e
Merge "Add media viewer [alpha]"