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>

routers/web/misc/markup.go

@@ -0,0 +1,44 @@
+// Copyright 2014 The Gogs Authors. All rights reserved.
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package misc
+
+import (
+	"net/http"
+
+	"code.gitea.io/gitea/modules/context"
+	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/modules/web"
+	"code.gitea.io/gitea/routers/common"
+)
+
+// Markup render markup document to HTML
+func Markup(ctx *context.Context) {
+	// swagger:operation POST /markup miscellaneous renderMarkup
+	// ---
+	// summary: Render a markup document as HTML
+	// parameters:
+	// - name: body
+	//   in: body
+	//   schema:
+	//     "$ref": "#/definitions/MarkupOption"
+	// consumes:
+	// - application/json
+	// produces:
+	//     - text/html
+	// responses:
+	//   "200":
+	//     "$ref": "#/responses/MarkupRender"
+	//   "422":
+	//     "$ref": "#/responses/validationError"
+
+	form := web.GetForm(ctx).(*api.MarkupOption)
+
+	if ctx.HasAPIError() {
+		ctx.Error(http.StatusUnprocessableEntity, "", ctx.GetErrMsg())
+		return
+	}
+
+	common.RenderMarkup(ctx, form.Mode, form.Text, form.Context, form.FilePath, form.Wiki)
+}