[MET-226] Adding filters to make searching for posts easier. Todo: passing the filter options to the search-as-you-type
Clean up advanced filters section.

Also don't output if no categories or posts with categories.
i18n: make moar strings translatable
ob_* functions not necessary here
Better validation for date and category filters and more accurate errors when building dropdown of posts
Fix date and cat filters for autocomplete.

Also, consolidate some dupe code for validating cat/date values.
Back-and-forwards compat with jQuery UI 1.8 and 1.9.

The latter uses "ui-" prefix for data attributes.
Make the show/hide toggle use the pointy cursor
More elegant swapping of hide/show labels (use one elem + attributes instead of two).
Add a "Filter" label to make the dropdowns more obvious.
The filters aren't very advanced.
Fix the infinite and multiplexing scroll of doom when removing a post.