Add link to Epic modal trigger button

What does this MR do and why?

#483099

Add link to new Epic page to create modal

Allow opening the new epic page in a new tab

Changelog: changed

How to test

0. Ensure namespace_level_work_items & work_item_epics is enabled
1. Go to epics list page
2. Ctrl/cmd click on New epic button (or right click > open in new tab); verify it opens in background tab
3. regular Click on New epic button and verify modal opens
4. Repeat steps 2 & 3 for New epic button from + menu in left sidebar
5. In modal, click on "Open in full page" link (next to title)