Break Jira connect app into multiple page components

What does this MR do and why?

Relate to #346089 (closed), this MR attempts to improve the architecture of the Jira Connect app with the following changes:

• Create a pages directory.
• Create pages/subscriptions.vue and pages/sign_in.vue.
• Pull necessary code out of components/app.vue and put into respective page components.
• Update components/app.vue to switch between each page depending on whether the user is signed in.

There are no user-facing changes.

Screenshots or screen recordings

Screenshots demonstrate that the user experience hasn't changed.

How to set up and validate locally

Set up guide: https://docs.gitlab.com/ee/development/integrations/jira_connect.html.

Setting up the Jira Connect app is quite an involved process. If you'd like to test manually, let me know and we can organize a sync demonstration call.

Alternatively, you can manually update the code to render the app component in a page you are more familiar with!

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 #346089 (closed)