Fix blogpost metadata

To make use of the option to filter on tags in the blog posts index, I cleaned up some of the metadata. Also to prepare for the possibility to filter by author. i.e.:

• capitalise tags
• when similar tags, choose one version and replace all others
• use author id when existing
• move tag to category when 'announcement'

Closes #535 (closed) as we make better use of the categories:

• news (time-sensitive)
• announcement (time-sensitive)
• tutorial (timeless)
• report (timeless)

I checked the change of category from news to announcement didn't break any internal link. It's possible that it breaks external links 🤷 but as announcements are quickly deprecated, I would say it's fine.