News categories to tags

From https://gitlab.hzdr.de/hifis/overall/hifis.net/-/issues/327#note_1412618

• Single category only for (hopefully) all posts, additional stuff moved to tags
  • This basically fixes the multi-category problem introduced in https://gitlab.hzdr.de/hifis/overall/hifis.net/-/merge_requests/514/diffs
• redirects from weird multi-category urls that may be in the landscape meanwhile
• Replace static links in new posts with jekyll auto-generated ones

Closes #327 (closed)