Download tab, basic download and preview

* Following design, use split button to display a pulldown
menu with possible image sizes. The download happens by
sending the special "download" parameter to the server.

* Offer link to preview image in the browser.

Change-Id: Ic9d895fead04c9128186c7376a0bb09f3596335c
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/79