[Reseller] Add message letting user know that they can't purchase storage, CI minutes, seat buttons on GitLab

Problem

As per this discussion, if the subscription was sold via reseller, we should not allow customers to purchase seats, storage or CI minutes.

customers-gitlab-com#4946 (closed) takes care of this on CustomersDot portal. We also need to do the same on GitLab by showing a message which informs the user why they can't purchase when they click on any of the below buttons.

Flow	Screenshot
Add seats (Usage quotas page)
Buy additional minutes (Usage quotas page)
Buy storage (Usage quotas page)
Purchase more storage (Usage quotas page)
Add seats (Billing page)

Proposal

When any of the above buttons are clicked, show a modal letting the user know that their subscription is in read-only mode and can't be modified.

Mockup: #378596[Message_when_reseller_customer_tries_purchasing.png]