Added min width to files table

Having a too small widths for the files table doesn't make sense as it
overlaps file actions with the files themselves and make them
unclickable, especially when viewed on mobile phone.

The compromise that this fix introduces is to have a minimum width that
will trigger horizontal scrolling, which will at least make it possible
to click files on mobile phones.
Merge pull request #5988 from owncloud/files-mobilequickfix

Added min width to files table