Open license deed in new tab

Not convinced this is a good thing (file description page still
opens in same window so it is somewhat inconsistent) but suddenly
leaving the lightbox to show the deed feels like a very unintuitive
behavior to me.

Change-Id: I2cca3e4241fd1bb2848c11cf425aa75aad8c4a30
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/472
Merge "Open license deed in new tab"