Migrate groups/epics/show.html.haml to GlTabs

What does this MR do and why?

This MR migrates the tab implementation of group epics from HAML / JQuery to Vue / JS. It's not a perfect solution as we're still directly manipulating the DOM from within Vue, but it's the most simple solution to deal with this migration and is definitely a step forward.

Screenshots or screen recordings

Before	After
Screen_Recording_2022-01-10_at_13.58.25	Screen_Recording_2022-01-10_at_13.57.57

How to set up and validate locally

• Browse to group epics
• Select an epic
• Toggle the tabs near the bottom of the page

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 #230790 (closed)