relations: configuring pagination

[gythaogg]

We can configure the pagination size of list views by setting paginate_by in for the table for list views.

In the detail view of an entity, (new) relations are paginated and we can modify the page size here by

1. adding a per_page parameter in the URL
   or
2. adding a per_page attribute to the Meta class of the EntityRelationsTable

It would be nice to have a uniform configuration method - either per_page in Meta class or paginate_by as an attribute of the table class for tables everywhere in APIS

[b1rger]

I'd suggest to drop the paginate_by attribute. The Meta.per_page attribute is part of django-tables2. I did not know that this Meta attribute exists when I introduced the paginate_by setting. I think it makes more sense to use the upstream solution than a custom one.

[gythaogg]

If I recall correctly, the Meta.per_page didn't work for the ListViews - (but it worked when I passed per_page via the request params in the query string).