Merge branch 'wiki-gfm-autocomplete' into 'master'

Fixed GFM autocomplete not loading on wiki pages

## What does this MR do?

Allows the GFM autocomplete to work on wiki pages.

## What are the relevant issue numbers?

Closes #19580 

See merge request !5132

CHANGELOG

@@ -35,6 +35,7 @@ v 8.10.0 (unreleased)
   - Added day name to contribution calendar tooltips
   - Make images fit to the size of the viewport !4810
   - Fix check for New Branch button on Issue page !4630 (winniehell)
+  - Fix GFM autocomplete not working on wiki pages
   - Fix MR-auto-close text added to description. !4836
   - Support U2F devices in Firefox. !5177
   - Fix issue, preventing users w/o push access to sort tags !5105 (redetection)

app/views/layouts/_init_auto_complete.html.haml

 - project = @target_project || @project
-- if @noteable
-  :javascript
-    GitLab.GfmAutoComplete.dataSource = "#{autocomplete_sources_namespace_project_path(project.namespace, project, type: @noteable.class, type_id: params[:id])}"
-    GitLab.GfmAutoComplete.cachedData = undefined;
-    GitLab.GfmAutoComplete.setup();
+- noteable_class = @noteable.class if @noteable.present?
+:javascript
+  GitLab.GfmAutoComplete.dataSource = "#{autocomplete_sources_namespace_project_path(project.namespace, project, type: noteable_class, type_id: params[:id])}"
+  GitLab.GfmAutoComplete.cachedData = undefined;
+  GitLab.GfmAutoComplete.setup();