Jira Connect App: Create `sign_in_button` component

What does this MR do and why?

Add a sign_in_button.vue component for the Jira Connect App.

This is Part 2 towards #339428 (closed).

Iteration	MR
1. Add add_namespace_button component	!73613 (merged)
2. Add add_sign_in_button component	🚀 This MR
3. ...	TBC

In preparation for a layout re-organisation (#339428 (closed)), this commit abstracts the sign in button into its own Vue component. Relevant tests have been added/updated.

There are no user-facing changes in this MR.

Screenshots or screen recordings

Recording demonstrates that the UX hasn't changed: once the user signs in, they can click a button to return to their Jira site.

jira-sign-in-button

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.