Allow user to edit group

This provides the group edit page, part of the paid signup flow (gitlab-org/growth/product#87 (closed))

Design desktop: https://gitlab-org.gitlab.io/gitlab-design/hosted/kcomoli/growth%2387-gitlab-com-purchase-experience--specs-preview/#artboard19

Design mobile: https://gitlab-org.gitlab.io/gitlab-design/hosted/kcomoli/growth%2387-gitlab-com-purchase-experience--specs-preview/#artboard1

Desktop	Mobile

There is no changelog, because the feature is behind a feature flag.

To test this feature in development setup:

• Sign in, visit http://localhost:3000/groups/new and create a new group
• visit http://localhost:3000/-/subscriptions/groups/#{group.path}/edit

Closes gitlab-org/growth/engineering#96 (closed)