Add strikethrough button to markdown editor (#36087)

Fixes: https://github.com/go-gitea/gitea/issues/36086


![strike](https://github.com/user-attachments/assets/984e36db-6fa8-4054-9794-aa54bc642354)

options/locale/locale_en-US.ini

@@ -215,6 +215,7 @@ more = More
 buttons.heading.tooltip = Add heading
 buttons.bold.tooltip = Add bold text
 buttons.italic.tooltip = Add italic text
+buttons.strikethrough.tooltip = Add strikethrough text
 buttons.quote.tooltip = Quote text
 buttons.code.tooltip = Add code
 buttons.link.tooltip = Add a link

templates/shared/combomarkdowneditor.tmpl

@@ -43,6 +43,7 @@
 			<div class="markdown-toolbar-group">
 				<md-bold class="markdown-toolbar-button" data-tooltip-content="{{ctx.Locale.Tr "editor.buttons.bold.tooltip"}}">{{svg "octicon-bold"}}</md-bold>
 				<md-italic class="markdown-toolbar-button" data-tooltip-content="{{ctx.Locale.Tr "editor.buttons.italic.tooltip"}}">{{svg "octicon-italic"}}</md-italic>
+				<md-strikethrough class="markdown-toolbar-button" data-tooltip-content="{{ctx.Locale.Tr "editor.buttons.strikethrough.tooltip"}}">{{svg "octicon-strikethrough"}}</md-strikethrough>
 			</div>
 			<div class="markdown-toolbar-group">
 				<md-quote class="markdown-toolbar-button" data-tooltip-content="{{ctx.Locale.Tr "editor.buttons.quote.tooltip"}}">{{svg "octicon-quote"}}</md-quote>