Draft: Create add seats modal for subscriptions with limited access

What does this MR do and why?

Related to [Reseller] Show modal to let users know they ca... (#409752 - closed)

Introduces a modal to show when subscriptions are either reseller managed or ramp subscriptions behind the limited_access_modal FF.

Screenshots or screen recordings

Group usage quota Add seats
Ramp subscription
Managed by reseller

How to set up and validate locally

• Create a ramp subscription

• Create a reseller subscription

• Create a subscription without either reseller or ramp attributes

• Run rails c and then `Feature.enable(:limited_access_modal)

For both ramp + reseller subscriptions:

• Link it to a GL namespace either in the CDot UI (subscriptions/SUBSCRIPTION_NAME/edit?transaction=change_gitlab_group) or via the rails console
• In GL, navigate to /groups/GROUP_NAME/-/usage_quotas#seats-quota-tab
• Verify that Add seats button appears as usual
• Verify that clicking the button displays the modal
  • By default, it will show the modal text for a reseller subscription because we're temporarily stubbing the limitedAccessReason
  • To see the ramp subscription text, you can modify this line to return RAMP_SUBSCRIPTION
• Verify that closing out of the modal functions properly

For the regular subscription:

• Link it to a GL namespace either in the CDot UI (subscriptions/SUBSCRIPTION_NAME/edit?transaction=change_gitlab_group) or via the rails console
• In GL, navigate to /groups/GROUP_NAME/-/usage_quotas#seats-quota-tab
• Verify that Add seats button appears as usual
• Verify that clicking the button takes you to CDot to add seats

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.