Fine-tune ACM CSS stylesheet

[clange]

• move the ACM CSS out of the fiduswriter.gesis.org database into the Fidus Writer repository
• compare the layout of some papers written in Fidus Writer, and displayed using the ACM CSS, to some real papers in ACM layout
• for fixing things, take inspiration from https://dokie.li/media/css/acm.css (developed by @csarven – Hi, @csarven, just FYI, we are also planning ACM-style output in Fidus Writer)
  • @sadeghiafshin also mentioned an official CSS file available from ACM – if that's not the one we have already, then that CSS file of ACM may also serve as a source of ideas.