make sidebar collapsible
When someone has a full calendar, the sidebar added in 43 effectively duplicates the events already in the week view to the agenda. Therefore, it might be for some users (including me ^^) just filling the screen with it most times not being interacted with / not used. In such circumstances, it would be helpful to not have it constantly visible. By making it collapsible, the sidebar can function as a date selector for those use cases, where an always visible agenda view is not needed(/wanted).
related to #606
Edited by Markus Göllnitz