[Reseller] Show modal to let users know they can't add seats on GitLab for reseller subscriptions

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.

#409750 (closed) will handle storage and minute buttons.

Flow	Screenshot
Add seats (Usage quotas page)
❌ Add seats (Billing page) this flow is not valid anymore, we don't show the Add seats button for reseller subscription

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]

Description	MR	Status
Update CDot to expose eligible_to_buy_ci_minutes and eligible_to_add_storage? as fields in subscription_type.rb	https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/7292	workflowcomplete
Send limited_access_reason to GL	https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/7329	workflowin dev
Create read-only modal in GL & Trigger modal from usage quotas page	!120047 (closed)	workflowin dev
Consume new endpoint with limited_access_reason in GL modal		workflowblocked Depends on https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/7329
Trigger modal from billing page		workflowblocked Depends on !120047 (closed)

Availability & Testing

Specs should be included in this change including - FE specs of the display of the modal, and at least two feature specs, 1x validating that the modal shows during the appropriate conditions.

• Who sees this?
• When? What other conditions?