Dashboards with "group" permission can't be seen by superusers not in the group

[toolness]

One confusing thing I noticed on my Django deployment is that when a coworker created a dashboard and assigned it a group permission, my own superuser account couldn't see it. It turned out that this was because my superuser wasn't explicitly a member of the specified group, but I had simply assumed that because I was a superuser, I was implicitly a member of all groups.

Is this intentional behavior? If not, I could submit a PR that allows superusers to see such dashboards.