handle forthcoming papers

[Tobias Gerstenberg]

• show forthcoming papers that are either submitted, accepted, or in press at the top of the publication list
• instead of the date, display the respective text instead
• how to handle this:
  • categorize these papers as Forthcoming (i.e. use value 3 in publication_types as stated in in config.toml line 140)
  • then use an if-statement in homepage/layouts/partials/publication_li_apa.html (line 8) to either display the year, or (if publication type is 3) display the text instead