Add "Reference" section to Issue view sidebar (go-gitea#19609)

* Add "Reference" section to Issue view sidebar

* Removed unneeded tailored CSS, added Fomantic UI classes

* Removed tailored CSS in favour of HTML-only approach

Kindly provided by @Gusted

* Added i18 translation key for reference link string

* Corrected spelling of "References:" to "Reference:" for reference link text

* Removed `Issue.ReferenceLink` in favour of a local template variable

options/locale/locale_en-US.ini

@@ -1480,6 +1480,7 @@ issues.content_history.created = created
 issues.content_history.delete_from_history = Delete from history
 issues.content_history.delete_from_history_confirm = Delete from history?
 issues.content_history.options = Options
+issues.reference_link = Reference: %s
 
 compare.compare_base = base
 compare.compare_head = compare

templates/repo/issue/view_content/sidebar.tmpl

@@ -568,6 +568,15 @@
 			{{end}}
 		{{end}}
 
+		<div class="ui divider"></div>
+		<div class="ui equal width compact grid">
+			<div class="row ac">
+				{{$issueReferenceLink := printf "%s#%d" .Issue.Repo.FullName .Issue.Index}}
+				<span class="text column truncate">{{.i18n.Tr "repo.issues.reference_link" $issueReferenceLink}}</span>
+				<button class="ui two wide button column p-3" data-clipboard-text="{{$issueReferenceLink}}">{{svg "octicon-copy" 14}}</button>
+			</div>
+		</div>
+
 		{{if and .IsRepoAdmin (not .Repository.IsArchived)}}
 			<div class="ui divider"></div>
 			<div class="ui watching">