Display image description/license/metadata in gallery view.

- Added pinch-zoom / scroll / fling functionality by using the PhotoView
  library (added to gradle and credits).
- Displaying image description, credit, and license on top of the image.
- Created a pop-up dialog of Exif and miscellaneous metadata accessible
  from overflow menu.
- Added option to navigate to the File page from overflow menu.
- Made links clickable in descriptions and metadata items.
- Refactored the Wikipedia Zero interstitial dialog (and removed the
  associated Bus event), so that activities don't have to individually
  implement the interstitial handler.

Change-Id: I983d84a76a96cb57e35ad33db2048d385e47b253