Fix up all the diff viewer anchor code.

The diff viewer anchor code was in a semi-working state. There were
still calls to the now-removed 'gotoAnchor', which errored out, and
there was a hacky global function for the individual DiffReviewableViews
to scroll anchors.

Now the views handle all anchor click events. DiffReviewableView
triggers events when the file header or a chunk is clicked, and
DiffViewerPageView reacts to that. The PageView also handles all clicks
from the index that comes before the diff.

Tested all anchors manually.

Unit tests pass.