Add infinite scrolling to the diff viewer.
When working with long diffs, it's common for the diffs to be split up across

several pages. This can end up being a pain to review, as it means having to

deal with manual pagination.
This change replaces the pagination to scroll loading. When user scroll to the

bottom of the page, diffs will be loaded automatically without having to click

anything.
First bind a listening function to the window to check the position. And when

it detect scroll bar touch the bottom, it will active the loading function to

load next diffs. The loading function use ajax to load pagination's diffs, and

append to the current pagination. Similar to the set, when update the diffs,

properties should update one by one.
Remove the pagination and page selection, and modify the description of 'paginate

by' and 'paginate orphans'.

Passed all js-tests.
Manual tests with a 25 diff files review request.

Paginate by 2, and paginate orphans is 10.
When first load the pages, the page loads 2 diffs, and scroll to the bottom, it

loads 2 another difss. At last, the page loads 10 diffs.