Write the code to have submenus on the sidebar

Accordion based navigation could do the job. For example http://media02.hongkiat.com/css-based-accordion/demo/index.html and http://tympanus.net/Tutorials/CSS3Accordion/index2.html .Both are CSS-only, no javascript. They could be easily styled to match the look and feel of the current menu. Inside each expanding panel we could place more links.

Feature Branch: web/7257-sidebar-submenus

Parent Task: #7214

Original created by @sajolida on 7257 (Redmine)