Add marker parameter to image requests coming from MediaViewer

This will be needed by Erik Zachte for compiling per-file image view data.
Since Media Viewer does preloading, it skews the HTTP request-based
statistics. By marking image requests coming from Media Viewer,
it allows us to remove that bias.

Change-Id: Iac8e7770c1a379691547de4b6d47b7d54467f54d
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/1002
Merge "Add marker parameter to image requests coming from MediaViewer"