MVC: Show transparent warnings when adding a user to a group will incur a charge

Problem to solve

Customers are not informed when they are about to go over their purchased seat count and incur a charge, this can cause unwanted surprises at renewal/true-up time.

Proposal

Add transparent messaging when they add a user to warn them that it will take them over their purchase seat count. Much of this UX/design work has already been completed here for self-managed and will be slightly different for GitLab.com. We should make sure these warnings are shown before the user is saved.

Iteration 1 (MVC): Better communication to users

What problem are we solving?	Proposed solution
Subscription seat information is only accessible to group owners/admins, but other user roles can also add users to paid seats, impacting billing.	Add subscription seat information on the members page for the instance/group and its subgroups, visible to all users who can add members to paid groups.
When a user is added to a paid group/instance that would put the group/instance over its seat count, it is not clear that adding that user will incur charges.	When a user is being added and it would increase the subscription's Seats owed/Users over license number, show a modal and ask the user to confirm that they would like to add a seat to their subscription.

Design

• See the full flow documented in Mural.
• See the design tab for designs
• Sketch measure for specs