Refactor markup and pdf-viewer to use new init framework (#33772)

1. Add some "render-content" classes to "markup" elements when the
content is rendered
2. Use correct "markup" wrapper for "preview" (but not set that class on
the tab)
3. Remove incorrect "markup" class from LFS file view, because there is
no markup content
    * "edit-diff" is also removed because it does nothing
5. Use "initPdfViewer" for PDF viewer
6. Remove incorrect "content" class from milestone markup
7. Init all ".markup" elements by new init framework

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/shared/combomarkdowneditor.tmpl

@@ -81,7 +81,7 @@
 			}
 		</script>
 	</div>
-	<div class="ui tab markup" data-tab-panel="markdown-previewer">
+	<div class="ui tab" data-tab-panel="markdown-previewer">
 		{{ctx.Locale.Tr "loading"}}
 	</div>
 	<div class="markdown-add-table-panel tippy-target">