Update viewer.css (#117)
Update viewer.css (#117)

Add one pixel to messages div width calculation to address issue 113
Tested on Ubuntu 18.04.3 64-bit
Firefox 71 (64-bit)
Chrome 79 (64-bit)
Vivaldi 2.9.1705.41 (64-bit)
hfaran/slack-export-viewer,hfaran/slack-export-viewer