Show warning when adding user to group that incur a charge

What does this MR do?

It shows a warning dialog when adding a user to a group will incur a charge. This happens when the subscription reaches (or about to reach) its seat limit.

The same approach applies to child groups if the parent have a subscription.

Screenshots

taken at 7849bbdc, 2020-02-10.

Subscription Grid	Confirmation Alert

To test the changes

Subscription grid

• Have a group where you have permission to manage its members
• Navigate to a group's members page
• Notice the grid at the top of the members' page

Confirmation modal

• Have a group with a subscription
• Have a users count equal or above the subscription's seats limit
• Add a user to the group
• The modal should appear blocking the user addition without confirmation

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #35683 (closed)