Introduce $args param to BP_Groups_Group constructor and groups_get_group()

This argument array can contain the following toggles:
- update_meta_cache (pre-fetch all group metadata for the queried group)
- populate_extras (pre-fetch last activity, admin/mod lists, and current user
  access/membership status in queried group)

Both items default to false, for backward compatibility

Allowing these pre-fetch toggles to be set to false allows us to avoid large
numbers of unnecessary database queries in certain cases, such as when
secondary avatars are populated on activity directories.

See #5398

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7898 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
When fetching the group in bp_get_activity_secondary_avatar(), don't get group extras

We have no need for membership info, etc, so we avoid a number of queries here.

See #5398

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7899 cdf35c40-ae34-48e0-9cc9-0c9da1808c22