Add support for bulk editing epics

What does this MR do and why?

Adds support for bulk editing epics (i.e. ability to apply labels to multiple epics at once) from the epics list page. This feature is already available in HAML epics list, but we're adding this to Vue implementation of Epics list which was added in !46769 (merged) and is currently behind feature flag vue_epics_list (see #276189 (closed)).

Screenshots or screen recordings

Bulk Editing Epic Labels
Screen_Recording_2021-12-14_at_2.21.12_PM

How to set up and validate locally

• Enable vue_epics_list through Rails console in any of the group within GDK, eg; Feature.enable(:vue_epics_list, Group.find_by_full_path('gitlab-org'))
• Visit Epics list page for the same group, eg; http://gdk.test:3000/groups/gitlab-org/-/epics
• Click on Edit epics button from above the list.
• Once sidebar on the right is revealed along with checkboxes next to each epic, select a couple of epics and open labels dropdown to select labels and click Update all.
• Page should refresh with selected labels applied on selected epics.

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.