Fixes broken labels in file explorer and manage files view.

Adds a new sass mixin "label" and uses it for single line fixed-width labels.
Makes files and entries in Files Explorer scrollable via overflow: auto.

File explorer is always full height. Some changes in html and css to make sure the height of the containing div is correct.
Merge pull request #37 from kioopi/file_explorer_fixes

Fixes for the "Wiederverwenden" File Explorer