[GITEA] Make reference URL absolute

- Backport of #2100
- Make the reference URL in the "Reference in New issue" feature
absolute again as it wouldn't render as a link otherwise.
- Adds integration test.
- Regression by 769be877f2
- Resolves #2012

(cherry picked from commit c74bae2897)

templates/repo/issue/view_content/context_menu.tmpl

@@ -5,9 +5,9 @@
 	<div class="menu">
 		{{$referenceUrl := ""}}
 		{{if .issue}}
-			{{$referenceUrl = printf "%s#%s" .ctxData.Issue.Link .item.HashTag}}
+			{{$referenceUrl = printf "%s#%s" .ctxData.Issue.HTMLURL .item.HashTag}}
 		{{else}}
-			{{$referenceUrl = printf "%s/files#%s" .ctxData.Issue.Link .item.HashTag}}
+			{{$referenceUrl = printf "%s/files#%s" .ctxData.Issue.HTMLURL .item.HashTag}}
 		{{end}}
 		<div class="item context js-aria-clickable" data-clipboard-text-type="url" data-clipboard-text="{{$referenceUrl}}">{{ctx.Locale.Tr "repo.issues.context.copy_link"}}</div>
 		{{if and .ctxData.IsSigned (not .ctxData.Repository.IsArchived)}}