Jira Connect app: Use new Subscriptions API

What does this MR do and why?

⚠ this is behind the :jira_connect_oauth feature flag. This MR addresses 2 inter-related issues:

• #325278 (closed) - uses the new Subscriptions API endpoint when a user adds a subscription (backend introduced in !86675 (merged))
• #359834 (closed) - moves user data (and user access token) into Vuex

Screenshots or screen recordings

Kapture_2022-05-12_at_15.51.53

How to set up and validate locally

1. Set up Jira and the GitLab for Jira app: https://docs.gitlab.com/ee/development/integrations/jira_connect.html#install-the-app-in-jira
2. Enable the jira_connect_oauth feature flag.
3. Sign into your Jira Connect App.
4. Click "Add subscription" and click "link" to add a subscription.
5. Observe. The modal should close, and the subscriptions list should refresh without a page refresh.

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.