[#1519] Fix organization activity filter in dashboard

The old implementation had the same type for orgs and groups and when an
org was selected a 404 was returned because group_show was called.

I didn't go all the way down separating both, ie group_followee list
still returns both groups and orgs.
[#1519] Separate group / org follow actions
[#1519] Fix test
Merge pull request #1519 from ckan/1519-fix-orgs-in-dashboard

Activity Stream from: Organization Error group not found