Add "Imported" badge to Epic header and sticky header

What does this MR do and why?

This is not shown at the moment as the backend is not providing any data.

Here is the list of visual indicator MRs for "Imported" badge:

MR	Status
Issues: !149876 (merged)	✅ Merged
Merge requests: !150095 (merged)	✅ Merged
Epics: !150266 (merged)	👈 You are here

Changelog: changed

EE: true

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Section	Before	After
MR header
MR sticky header

How to set up and validate locally

1. Go to Group > Plan > Epics and select any epic. For example, http://127.0.0.1:3000/groups/flightjs/copy/gitlab-instance-2/-/epics/5.
2. Apply the following diff to show the badge:

diff --git a/ee/app/helpers/ee/issuables_helper.rb b/ee/app/helpers/ee/issuables_helper.rb
index 84566200b3a2..edfa4adcb3ae 100644
--- a/ee/app/helpers/ee/issuables_helper.rb
+++ b/ee/app/helpers/ee/issuables_helper.rb
@@ -24,6 +24,7 @@ def issuable_initial_data(issuable)
 
       if parent.is_a?(Group)
         data[:confidential] = issuable.confidential
+        data[:imported] = true
         data[:epicLinksEndpoint] = group_epic_links_path(parent, issuable)
         data[:epicsWebUrl] = group_epics_path(parent)
         data[:fullPath] = parent.full_path

Related to #456082 (closed)