Add "Unique Groups Enrolled" to "By Decision Point" section on experiment data tab

[zackcl]

We're currently not showing the group enrollment info on the experiment details page. It would be useful to add "Unique Groups Enrolled" below "Unique Users Enrolled" to the "By Decision Point" section on experiment data tab, when the experiment's assignment unit is "Group".

[danoswaltCL]

QA: Dan