Endless scrolling

When JavaScript is enabled, "Next Page" button below sticky note board
is eliminated and AJAX instead automatically loads more content on the
same page. Selector ".board" and its children are no longer unordered
lists and list items, but instead a division with more divisions to
simplify AJAX code while maintaining HTML validity. Print an input tag
instead of an anchor tag at the end of a endless scroll content request.
Add ids to form for loading the next page and start input for DOM
purposes.
Centered masonry and improved endless scrolling

Use Isotope instead of Masonry for bigger feature set. Consulted
Isotope's Centered Masonry demo to implement centered masonry on post
board page. Rely on Isotope's onLayout callback to determine when to
conditionally load more content; this improves over the last
implementation in that the function is executed after Isotope finishes
reflowing masonry, so $(document).height() returns the expected value.