Ability to add a group to another group (BB-11222)

At the moment when adding a members to a group, you can add with an email address or a username.

Ideally we would like you to be able to add other groups.

Use case:

Devs:
bob
sam

Testers:
joe
liz

All:
<Testers>
<Devs>

We have a lot of repos and quite a lot of users, we would like to be able to manage meta groups to allow some repos to easily have essentially global read permissions to our users, but also have some fine grained groups. I don't want to have to add a new member of staff to lots of individual groups.