Move Jira Connect header from HAML to Vue

What does this MR do and why?

Moves most of the Jira Connect HAML to Vue. In doing this, we also slightly refactor the user-link styling.

Screenshots or screen recordings

before	after

How to set up and validate locally

Set up the Jira Connect app with Gitpod, and observe the rendered UI: https://docs.gitlab.com/ee/development/integrations/jira_connect.html

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.