places footer relatively and remove bottom padding

alternative footer implementation that should prevent the footer from hiding the menu. It is based on [this tutorial](http://www.lwis.net/journal/2008/02/08/pure-css-sticky-footer/)

Additionally, it removes the 155px padding at the bottom which produces a lot of empty space. So e.g. https://github.com/opf/openproject/pull/1601 would no longer be necessary. However, some pages currently rely on this padding (e.g. work package, backlogs) and thus merging this without taking a look at fixing this might be hasty.