Clean up scrolling and infinite stream

Simplify our efforts around scrolling and fix the scrollbars on the
stream.

- Scrollbar had crept inside the content on a previous change such
  that scroll bars for the stream were not all the way to the edge
  of the screen. Refactoring the duties of #wrapper and .content.
- Drop the mousewheel trapping. I tried and looked and researched
  and we can't do this reliably on touch or anything else. It didn't
  seem necessary or worth the dependency on jquery.mousewheel or
  even obviously a good experience. Not worth the code.
- Fix infinite scroll to use its directive element rather than the
  $window, make its code less jquery, and give it a buffer to smooth
  the experience a bit more.

Fix #1360
Remove jquery.mousewheel from karma.config
Merge pull request #1373 from hypothesis/1360-scrolling

Clean up scrolling and infinite stream