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/common/markup.go

@@ -0,0 +1,92 @@
+// Copyright 2014 The Gogs Authors. All rights reserved.
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package common
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/markup/markdown"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/util"
+
+	"mvdan.cc/xurls/v2"
+)
+
+// RenderMarkup renders markup text for the /markup and /markdown endpoints
+func RenderMarkup(ctx *context.Context, mode, text, urlPrefix, filePath string, wiki bool) {
+	markupType := ""
+	relativePath := ""
+
+	if len(text) == 0 {
+		_, _ = ctx.Write([]byte(""))
+		return
+	}
+
+	switch mode {
+	case "markdown":
+		// Raw markdown
+		if err := markdown.RenderRaw(&markup.RenderContext{
+			Ctx:       ctx,
+			URLPrefix: urlPrefix,
+		}, strings.NewReader(text), ctx.Resp); err != nil {
+			ctx.Error(http.StatusInternalServerError, err.Error())
+		}
+		return
+	case "comment":
+		// Comment as markdown
+		markupType = markdown.MarkupName
+	case "gfm":
+		// Github Flavored Markdown as document
+		markupType = markdown.MarkupName
+	case "file":
+		// File as document based on file extension
+		markupType = ""
+		relativePath = filePath
+	default:
+		ctx.Error(http.StatusUnprocessableEntity, fmt.Sprintf("Unknown mode: %s", mode))
+		return
+	}
+
+	if !strings.HasPrefix(setting.AppSubURL+"/", urlPrefix) {
+		// check if urlPrefix is already set to a URL
+		linkRegex, _ := xurls.StrictMatchingScheme("https?://")
+		m := linkRegex.FindStringIndex(urlPrefix)
+		if m == nil {
+			urlPrefix = util.URLJoin(setting.AppURL, urlPrefix)
+		}
+	}
+
+	meta := map[string]string{}
+	if ctx.Repo != nil && ctx.Repo.Repository != nil {
+		if mode == "comment" {
+			meta = ctx.Repo.Repository.ComposeMetas()
+		} else {
+			meta = ctx.Repo.Repository.ComposeDocumentMetas()
+		}
+	}
+	if mode != "comment" {
+		meta["mode"] = "document"
+	}
+
+	if err := markup.Render(&markup.RenderContext{
+		Ctx:          ctx,
+		URLPrefix:    urlPrefix,
+		Metas:        meta,
+		IsWiki:       wiki,
+		Type:         markupType,
+		RelativePath: relativePath,
+	}, strings.NewReader(text), ctx.Resp); err != nil {
+		if markup.IsErrUnsupportedRenderExtension(err) {
+			ctx.Error(http.StatusUnprocessableEntity, err.Error())
+		} else {
+			ctx.Error(http.StatusInternalServerError, err.Error())
+		}
+		return
+	}
+}