Add FLA 2.0 from contributoragreements.org

This introduces the FLA 2.0, while preserving the 1.3 documents that were in use until 2021.

The FLA 2.0:

• new text, taken unchanged from the contributoragreements.org website; there is a wizard that takes in input and spits out a markdown document, which is what we've used.
• the relicensing policy is no longer conditioned to a KDE-specific FRP; the FRP was massively out-of-date with actual practice in the KDE codebase (e.g. the license policy was updated but the FRP wasn't) and full of deprecated language and broken URLs. Rather than fix that, use the intersection of FSF and OSI which is the default FLA language anyway.

Note that all the already-signed FLA documents (1.3 era) remain valid and unchanged. If you signed a 1.3 document, then the corresponding 1.3 FRP applies. Only new signatures on a 2.0 document use the expanded (I'd say "more realistic") relicensing language.

There's an upcoming FLA 2.1 from the same upstream, which we could import in largely the same way.