do not store GroupOrderArticles with zero quantity and tolerance

In `GroupOrderArticle` there is a scope `ordered` which means both "those articles that have been ordered", as well as "ordered by group name". In other models, `ordered` is used as the latter.

Cleaning this up, I could not find a reason why to keep `GroupOrderArticle`s where all quantities are zero. These records are currently the bulk of the records in this table, and consume quite some computation and disk access time (reference to foodcoops#49).

I've tested this on a copy of a production database with 1117655 `GroupOrderArticles`, which were reduced to a mere  12372 after the migration (the migration took 5 seconds on my development machine).

@bennibu as this affects the ordering algorithm, would you please comment if this is a good thing to do?

p.s. in the show group order page, articles not ordered are still presented (unless hidden by javascript)