Add deprecation notice to news and events feeds

[NickSto]

This adds a warning to the top of pages like /news, /events, and their member site equivalents. It alerts visitors of the move to galaxyproject.org (#1053).

When we start the switchover, we'll have all the recent news and events on .org but the older posts will still be on .eu. So we'll leave the news/events lists up on .eu, but add this notice to readers know that they need to go to .org to find new updates.

See here for details about the plan for different pages: galaxyproject/galaxy-hub#1480 (comment)

Here's a sample of how the notice looks:

[bgruening]

@NickSto just to understand your idea here. This will give users a warning that content has moved. But the link https://galaxyproject.eu/news will still work correct, because it will be redirected to https://galaxyproject.org/eu/news.
So the only case where people actually see this warning is if they visit the github-page URL?

[NickSto]

@bgruening Yes, galaxyproject.eu/news will still work and visitors will be able to remain on that page (with this warning). They won't be automatically (301) redirected to galaxyproject.org/eu/news/ (since it's not listed in usegalaxy-eu/infrastructure-playbook#421). We need them to be able to remain on galaxyproject.eu/news because it contains historical posts we don't have on .org yet.

[bgruening]

But galaxyproject.eu/news/ is redirected, or should, because we redirect the entire domain of galaxyproject.eu to galaxyproject.org/eu/ - (the URL stays). With that, we also redirect the news and post section.

Maybe I'm missing something, or what I propose is not possible. What I'm trying is that the URLs stay as they are (for most of the sites) and the user does not see any big change.

[NickSto]

FYI, to make sure everyone's on the same page, I wrote out exactly what will happen to several example pages in #1053. Hopefully this clarifies the plan.