Fix work item /new page for groups

What does this MR do and why?

Fix base path for vue router when loaded from a Group page (url contains /groups/ at the start before fullPath)

Some refactoring for Work items at the group level - create epic fro... (#429127 - closed)

This doesn't fix:

• Title/breadcrumb on New workitem page is still #new
• Create mutation is missing some props

How to set up and validate locally

1. Visit any group or project work items pages such as http://gdk.test:3000/groups/flightjs/-/work_items/new or http://gdk.test:3000/flightjs/Flight/-/work_items/new
2. Create a work item