LDEV-2820: Replace lesson links placed one-by-one with dropdown menu.
Make dropdown menu initialisation code base on class attribute, not ID. Also make it work with dynamically loaded content by using jQuery live() method.
Add CSS file for the menu.
As the menu is a DIV and can not be displayed inline, unlike one-by-one lesson links, changes to lesson list layout were needed. The items of list are now table rows instead of paragraphs. This broke lesson sorting but it was fixed.