MVC: Show transparent warnings when adding a user to an instance 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

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 admin>users page and the members page for groups/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

Requirements

1. The modal (to confirm that the user is adding an additional user to their license) should not show up based solely on if the Active users number is greater than the Users in license number. It should only show up if the Maximum users/Licenses owed number is going to be increased by the addition of the user/users that are being added to the instance. If the user is already being counted as "using a license" because they're part of another group or project within the instance, it shouldn't trigger the modal.
2. The subscription usage widget should show up on the admin users page and members page for all groups and subgroups. The subscription widget should still be titled with the name/plan of the instance to which the subscription is applied.
3. If the instance is on a free plan with no subscription, that subscription usage component shouldn't show up at all.
4. The Maximum users and Licenses owed numbers in the UI should increment immediately after a user is added that would increase those values.
5. For Ultimate subscriptions (where Guest users are free), the add user modal should trigger when a Guest user is given a higher role and it would cause the Maximum users/Licenses owed number to increase.
6. For Ultimate subscriptions, users who are added to the instance but not added to any groups or projects are free. When one of those users is added to a group or project, the add user modal should trigger if it would cause the Maximum users/Licenses owed number to increase.
7. The Members page now allows the customer to add groups as well as individuals. When a group is invited via the Members page, the add users modal should trigger if the Maximum users/Licenses owed number is going to be increased by the addition of the user/users that are being added.