[Reseller] Hide `Buy new subscription` button for customers who have purchased subscription through reseller

Problem

For customers who have purchased subscriptions through resellers, we should hide the "Buy new subscription" button.

Note: Subscription management buttons will be hidden for reseller subscriptions in this issue - #4946 (closed). In the current issue we need to tackle hiding the Buy new subscription button.

Proposal

1. If a customer has an active subscription that was purchased via a Reseller, they should not be able to buy a new subscription.
   • Add 30 days to the SubscriptionEndDate of Active (reseller) subscription to account for the renewal term
   • 30 days is based on https://about.gitlab.com/handbook/sales/field-operations/order-processing/#late-renewals
2. If a customer purchased via a reseller in the past, and no longer has an active (reseller) subscription, they should be able to buy a new subscription.

Implementation

MR	Description
https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/5925	Add managed_by_reseller attribute to subscription presenter output
https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/5926	Hide Buy new subscription button if any of the subscriptions have managed_by_reseller true

Result

Next steps (if any)

• @tgolubeva: Respond to https://gitlab.com/gitlab-com/support/ticket-attention-requests/-/issues/3095#note_1166598959 once this issue is closed.

How will we measure success?