Move "Add Namespace" button to GlEmptyState

What does this MR do and why?

This MR reorganizes the UI logic for the Jira Connect App. This is part-UI enhancement, part tech-debt cleanup.

To the user, the main change is that the Add Namespace button is now in the empty state when the user is signed in but has no subscriptions.

This restructuring also enables us to move some HAML markup to Vue.

This is Part 3 (the final part, excluding follow-ups) towards #339428 (closed).

Iteration	MR
1. Add add_namespace_button component	!73613 (merged)
2. Add add_sign_in_button component	!73618 (merged)
3. Move "Add Namespace" button to GlEmptyState	🚀 This MR

Screenshots or screen recordings

User signed out, no subscriptions ever (first time user)

Before	After

User signed out, no subscriptions (but has previously had subscriptions)

Before	After

User signed out, has subscriptions

Before	After

User signed in, no subscriptions

Before	After

User signed in, has subscriptions

Before	After

How to set up and validate locally

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

(if you'd like to test sync, let me know and we can set up a call 🙂)

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.