Add automated "this article might be deprecated" functionality

[andreasbovens]

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General proposal is to automatically add a "this article might be deprecated" type of notice (exact string TBD) to every article older than 1 year (exact time span TBD), unless otherwise indicated.

1. Jekyll looks at YAML metadata to see if there is an updated property with a date value that's less than 1 year ago. If it is older, insert notice; if it's not, don't insert notice. If there is no value, continue to step 2.
2. Jekyll looks at date in blog post file name: if there it's older, insert notice, if it's not, don't insert notice.

Once this is there, we can bring back more old articles.

[andreasbovens]

Also somewhat related: https://github.com/operasoftware/devopera/issues/18