Meets #14375: Footer is not fixed at the bottom after opening a taskboard

[``* `#14375` Footer is not fixed at the bottom after opening a taskboard``](https://www.openproject.org/work_packages/14375)
The footer is 55px tall, so I guess the 155px padding for div#main was intended to cover 100px for contextual menu and the remaining 55px for the footer, which is absolutely positioned and overlapping. Now I suppose we should set a 100px (or 105px for a little spacing) padding for the div#main to give it space for the contextual menu.