CategorySelect: prevent duplicate categories.
CategorySelect: lazy load article categories for front-end duplicate verification.
CategorySelect: update article page styles. Move some logic out of template and into controller.
Popover hint for adding categories.

Fix Sizzle exceptions for invalid expressions when trying to match against strings in getCategories.
Always uppercase the first letter of a category to properly match Mediawiki titles.
Implement add category button and popover hint for article pages.
Fix preserving the order in which categories are added.
Add i18n for add category popover hint.
Restyle article page according to new design specs.