Merge branch 'ee-55948-help-text-formatting-wiki' into 'master'

Port of 55948-help-text-formatting-wiki to EE

See merge request gitlab-org/gitlab-ee!11494

app/views/help/index.html.haml

 %div
 - if Gitlab::CurrentSettings.help_page_text.present?
-  = markdown(Gitlab::CurrentSettings.help_page_text)
+  .prepend-top-default.md
+    = markdown_field(Gitlab::CurrentSettings.current_application_settings, :help_page_text)
   %hr
 
 %h1

changelogs/unreleased/55948-help-text-formatting-wiki.yml

+---
+title: Format extra help page text like wiki
+merge_request: 26782
+author: Bastian Blank
+type: fixed