Begin moving to new DiffReviewable/DiffReviewableView objects.

This forms the base of the upcoming diff viewer rewrite. The older
DiffReviewable has moved into diffviewer/models/, and its fragment
loading functions have been loaded to operate partly on state created
during instantiation, instead of taking that state as parameters.

DiffReviewableView is what all the rendering code for a given file's
side-by-side diff will be handled by. Right now, it just has the code
for finding a table row for a given line number. The logic for that code
has remained the same, but has some cleanups (line number parsing has
been factored out into a function and var statements have been moved to
the top of the function).

Future changes will move more of the diff viewer code into here, piece
by piece.

Loaded a diff and saw the comment flags in all the right places.
They were still clickable and all, but that's expected since that code
didn't change.