Blog: 'Announcements' -> 'Releases'

[SethTisue]

references #1006

this PR reflects my understanding of what Darja and Adrien decided on recently, namely that everything is either Scala Highlights, or a release announcement, or a blog post

some blog posts were misfiled as release announcements so I recategorized them and added redirects

in the second commit I got rid of the old "news" category which we had stopped using years ago

oy what a mess

[SethTisue]

One thing that I didn't clean up in this round is that it seems random whether things are in /_posts or /blog/_posts. In a general way it seems the former is used for release announcements and the latter for everything else, but there are posts in the "wrong" directory in both directions.

It isn't clear to me that it even matters which directory something goes in. If so many posts are in the "wrong" directory but we've never noticed, then maybe it doesn't matter and only the category: declarations matter?

[SethTisue]

Another thing I didn't clean up except in a couple places is that many files declare post-type: announcement or post-type: blog. As far as I can tell, this does nothing and only category: matters.

There used to be code that looked at that attribute but that code was removed years ago (21b6d53).

[adpi2]

Thanks a lot for working on this!

I hope these categories are clearer and won't confuse anyone anymore.