group_dictize is very slow when the group has a lot of packages

`group_dictize` (and therefore the `group_show` action and the group page) is very slow when the group has a lot of packages. For example if a group has tens of thousands of packages, the group's page can take several seconds to render. The reason seems to be that `group_dictize` also dictizes each of the group's packages:

https://github.com/okfn/ckan/blob/682e6bc0a9fbc0a08448bdf53b793a8c9352916f/ckan/lib/dictization/model_dictize.py#L336

It would mean changing the API, but I propose the fix this by changing group dicts to contain a list of only the names of all the group's packages, not their full dicts.