Convert RB.Screenshot to Backbone.

Review Request #3947 — Created March 7, 2013 and submitted — Latest diff uploaded

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master

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Convert RB.Screenshot to Backbone.

This is a fairly trivial change to migrate the old screenshot object to
Backbone.js. The only tricky part of this is that I had to add an indirection
step to BaseResourceModel to ensure that the parent object is loaded before
calling url(), because if we don't have the parent links, we won't have a URL
for the object.
Tested setting screenshot captions and deleting screenshots. Ran js-tests.