Use VueX store for admin subscription component

What does this MR do and why?

This converts portions of the admin subscription components to a Vuex store for this follow up MR. This should have no functional changes, and is just to set up the functionality for the follow up MR.

Screenshots or screen recordings

How to set up and validate locally

• Navigate to /admin/subscription
• If you don't already have a linked subscription, set up a subscription within CustomersDot and get the activation code to link the subscription
• Verify that the subscription details section appears as expected
• Verify that the sync, manage, and remove buttons work as expected

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.

Related to #374938 (closed)