Bug 835506: Download Tab tests and copy changes

Adds unit tests for Download Tab's utils, decorators and views/urls. In some cases, modifies code to pass tests. The test suite includes new assertions in an extension of `TestCase` class and some Facebook-specific utility functions, some of which are adaptations of similar ones found in Affiliates.

This also changes the Download Tab's copy as per feedback from Mozilla. There are slight tweaks in the styles and markup to accommodate the changes in copy.

Will squash down to a single commit after review.