Jira Connect App: break `app/assets/javascripts/jira_connect/subscriptions/components/app.vue` into multiple components

Our Jira Connect Vue application is getting too big and complex. This makes contributing harder.

We should create components for each logical slice and re-compose them in app.vue

Proposal

• Create sign_in_page.vue and subscriptions_page.vue
  • Re-organize logic in app.vue to show these pages depending on userSignedIn state.
• Create component for the alert
• ...

Can draw inspiration from !67949 (diffs)

Availability & Testing

Existing coverage in spec/frontend/jira_connect/subscriptions/components/app_spec.js should support a refactor effort.