Fix leftcontent positioning. Ref #1255

@jancborchardt Using position:relative seems to do the trick.

@georgehrke Calendar will need to add `width:100%`  and remove `top: 3em;` on `fullcalendar`. Maybe also add `-moz-box-sizing:border-box; box-sizing:border-box;`

@DeepDiver1975 The mail app should remove the css for #leftcontent.

/cc @Raydiation