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/api/v1/misc/markdown.go

@@ -1,127 +0,0 @@
-// Copyright 2014 The Gogs Authors. All rights reserved.
-// SPDX-License-Identifier: MIT
-
-package misc
-
-import (
-	"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"
-	api "code.gitea.io/gitea/modules/structs"
-	"code.gitea.io/gitea/modules/util"
-	"code.gitea.io/gitea/modules/web"
-
-	"mvdan.cc/xurls/v2"
-)
-
-// Markdown render markdown document to HTML
-func Markdown(ctx *context.APIContext) {
-	// swagger:operation POST /markdown miscellaneous renderMarkdown
-	// ---
-	// summary: Render a markdown document as HTML
-	// parameters:
-	// - name: body
-	//   in: body
-	//   schema:
-	//     "$ref": "#/definitions/MarkdownOption"
-	// consumes:
-	// - application/json
-	// produces:
-	//     - text/html
-	// responses:
-	//   "200":
-	//     "$ref": "#/responses/MarkdownRender"
-	//   "422":
-	//     "$ref": "#/responses/validationError"
-
-	form := web.GetForm(ctx).(*api.MarkdownOption)
-
-	if ctx.HasAPIError() {
-		ctx.Error(http.StatusUnprocessableEntity, "", ctx.GetErrMsg())
-		return
-	}
-
-	if len(form.Text) == 0 {
-		_, _ = ctx.Write([]byte(""))
-		return
-	}
-
-	switch form.Mode {
-	case "comment":
-		fallthrough
-	case "gfm":
-		urlPrefix := form.Context
-		meta := map[string]string{}
-		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, form.Context)
-			}
-		}
-		if ctx.Repo != nil && ctx.Repo.Repository != nil {
-			// "gfm" = Github Flavored Markdown - set this to render as a document
-			if form.Mode == "gfm" {
-				meta = ctx.Repo.Repository.ComposeDocumentMetas()
-			} else {
-				meta = ctx.Repo.Repository.ComposeMetas()
-			}
-		}
-		if form.Mode == "gfm" {
-			meta["mode"] = "document"
-		}
-
-		if err := markdown.Render(&markup.RenderContext{
-			Ctx:       ctx,
-			URLPrefix: urlPrefix,
-			Metas:     meta,
-			IsWiki:    form.Wiki,
-		}, strings.NewReader(form.Text), ctx.Resp); err != nil {
-			ctx.InternalServerError(err)
-			return
-		}
-	default:
-		if err := markdown.RenderRaw(&markup.RenderContext{
-			Ctx:       ctx,
-			URLPrefix: form.Context,
-		}, strings.NewReader(form.Text), ctx.Resp); err != nil {
-			ctx.InternalServerError(err)
-			return
-		}
-	}
-}
-
-// MarkdownRaw render raw markdown HTML
-func MarkdownRaw(ctx *context.APIContext) {
-	// swagger:operation POST /markdown/raw miscellaneous renderMarkdownRaw
-	// ---
-	// summary: Render raw markdown as HTML
-	// parameters:
-	//     - name: body
-	//       in: body
-	//       description: Request body to render
-	//       required: true
-	//       schema:
-	//         type: string
-	// consumes:
-	//     - text/plain
-	// produces:
-	//     - text/html
-	// responses:
-	//   "200":
-	//     "$ref": "#/responses/MarkdownRender"
-	//   "422":
-	//     "$ref": "#/responses/validationError"
-	defer ctx.Req.Body.Close()
-	if err := markdown.RenderRaw(&markup.RenderContext{
-		Ctx: ctx,
-	}, ctx.Req.Body, ctx.Resp); err != nil {
-		ctx.InternalServerError(err)
-		return
-	}
-}