Editor preview support for external renderers (#23333)

Remove `[repository.editor] PREVIEWABLE_FILE_MODES` setting that seemed
like it was intended to support this but did not work. Instead, whenever
viewing a file shows a preview, also have a Preview tab in the file
editor.

Add new `/markup` web and API endpoints with `comment`, `gfm`,
`markdown` and new `file` mode that uses a file path to determine the
renderer.

Remove `/markdown` web endpoint but keep the API for backwards and
GitHub compatibility.

## ⚠️ BREAKING ⚠️

The `[repository.editor] PREVIEWABLE_FILE_MODES` setting was removed.
This setting served no practical purpose and was not working correctly.
Instead a preview tab is always shown in the file editor when supported.

---------

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/structs/miscellaneous.go

@@ -15,13 +15,41 @@ type SearchError struct {
 	Error string `json:"error"`
 }
 
+// MarkupOption markup options
+type MarkupOption struct {
+	// Text markup to render
+	//
+	// in: body
+	Text string
+	// Mode to render (comment, gfm, markdown, file)
+	//
+	// in: body
+	Mode string
+	// Context to render
+	//
+	// in: body
+	Context string
+	// Is it a wiki page ?
+	//
+	// in: body
+	Wiki bool
+	// File path for detecting extension in file mode
+	//
+	// in: body
+	FilePath string
+}
+
+// MarkupRender is a rendered markup document
+// swagger:response MarkupRender
+type MarkupRender string
+
 // MarkdownOption markdown options
 type MarkdownOption struct {
 	// Text markdown to render
 	//
 	// in: body
 	Text string
-	// Mode to render
+	// Mode to render (comment, gfm, markdown)
 	//
 	// in: body
 	Mode string