UX: Make group membership UI clearer (PR #8210)

The ‘automatically set primary group’ checkbox looked like it was associated with the email membership. In fact, it applies to all members who join the group. This commit moves it next to the ‘automatic trust level’ setting, and puts them both under an ‘Effects’ heading.

Before:

Screenshot 2019-10-17 at 16 56 15

After:

Screenshot 2019-10-17 at 16 53 10

GitHub

You’ve signed the CLA, davidtaylorhq. Thank you! This pull request is ready for review.