Limit packages returned by model_dictize.group_dictize

Related to #1569, `lib.helpers.featured_group_org` passes a `limits` object, with a `packages` parameter set to 2, in the context. This wasn't being used to limit the packages being returned, so this has been fixed.

The configuration docs stated that 2 groups (and 2 orgs) and 2 datasets would display on the homepage layouts. This has been corrected to state 1 group (and 1 org).