Use structured author data from EU

[NickSto]

usegalaxy-eu/website#1055 adds a lot of very useful structured data about the authors of European news posts. We should make use of it.

There are two issues currently in the way:

1. We currently don't make use of structured author data. authors is currently a raw string. Making use of the full data will probably have to wait for the Community Directory implementation to be decided (see #1551)
2. We'd already ported a number of .eu posts before the author data got added. We'll have to backport the data into those posts.
   • These posts were imported in: #1325, #1613, #1637, and #1646

For the rest of the news posts, I'm including the data in the key authors_structured as (likely) temporary measure. It's not even decided yet what the final name of the field will be once the Community Directory lands, but I imagine it'll be the same as our current one (authors). So until that field supports non-strings, I used authors_structured as essentially a temporary holding place for the data. The data in that field is well structured so that later it can easily be automatically changed to whatever the final schema will be.