Deduplicate security advisories by generating blog articles from the PGP messages

[TheLortex]

A solution for #779

Source of truth is PGP signed messages. The site generator reads them and automatically generates blog post articles according to the content. In particular is looks for the Announced field to set the blog post date, and the Affects field for blog post subject.

For consistency I have moved the advisories under the data/security folder. I have also edited the content of the advisories so that they have a consistent style, that is easier to parse. This means that the PGP signatures are not correct anymore, @hannesm you will have to update the signatures.

[hannesm]

Dear @TheLortex, thanks for this PR. But since PGP signatures carry a timestamp, I'm hesitant to re-sign all advisories without adding any information inside of them.

In addition, some of your formatting changes are done by the sign operation (such as adding trailing whitespaces, but also leading dash handling is special). Did you test that removing the signature and signing the raw data will lead to something your ad-hoc parser is able to use?

I would also refrain from moving the advisories to another random place once again - I don't see any benefits in doing that, but please elaborate why this is needed?