Add "Imported" badge to MR 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.
• Also fixed alignment in the MR sticky header to center instead of baseline.

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

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

Changelog: changed

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
MR sticky header (with ruler)

How to set up and validate locally

1. Go to any MR.
2. Apply the following diff:

diff --git a/app/helpers/merge_requests_helper.rb b/app/helpers/merge_requests_helper.rb
index 22abccf3dbd9..91ccf1a582b5 100644
--- a/app/helpers/merge_requests_helper.rb
+++ b/app/helpers/merge_requests_helper.rb
@@ -290,6 +290,7 @@ def sticky_header_data
       title: markdown_field(@merge_request, :title),
       isFluidLayout: fluid_layout.to_s,
       blocksMerge: @project.only_allow_merge_if_all_discussions_are_resolved?.to_s,
+      imported: true.to_s,
       tabs: [
         ['show', _('Overview'), project_merge_request_path(@project, @merge_request), @merge_request.related_notes.user.count],
         ['commits', _('Commits'), commits_project_merge_request_path(@project, @merge_request), @commits_count],
diff --git a/app/views/projects/merge_requests/_mr_box.html.haml b/app/views/projects/merge_requests/_mr_box.html.haml
index c98252ef63cc..d234777c94ed 100644
--- a/app/views/projects/merge_requests/_mr_box.html.haml
+++ b/app/views/projects/merge_requests/_mr_box.html.haml
@@ -1,3 +1,3 @@
 .detail-page-description.gl-pt-2.gl-pb-4.gl-display-flex.gl-align-items-baseline.gl-flex-wrap{ class: "#{'is-merge-request' if !fluid_layout}" }
-  .js-mr-header{ data: { project_path: @merge_request.project.path_with_namespace, hidden: @merge_request.hidden?.to_s, iid: @merge_request.iid, state: @merge_request.state } }
+  .js-mr-header{ data: { project_path: @merge_request.project.path_with_namespace, hidden: @merge_request.hidden?.to_s, imported: true.to_s, iid: @merge_request.iid, state: @merge_request.state } }
   = merge_request_header(@project, @merge_request)

Related to #456080 (closed)