Atom Feeds: Author names are not displayed properly

Currently, only the author's key (i.e. huste) given in the post metadata is displayed in the atom feed. The key is not resolved to the actual name of the author available from _data/hifis_team.yml.

The reason seems to be, that we're not using the default _data/authors.yml file. (see https://github.com/jekyll/jekyll-feed#author-information). As far as I can see, this file does not seem to be configurable for jekyll-feed.

Investigate possible solutions.

Atom feeds were introduced by !205 (merged).