Support relative paths to videos from Wiki pages (#31061)

This change fixes cases when a Wiki page refers to a video stored in the
Wiki repository using relative path. It follows the similar case which
has been already implemented for images.

Test plan:
- Create repository and Wiki page
- Clone the Wiki repository
- Add video to it, say `video.mp4`
- Modify the markdown file to refer to the video using `<video
src="video.mp4">`
- Commit the Wiki page
- Observe that the video is properly displayed

---------

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

modules/markup/html_node.go

@@ -0,0 +1,62 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markup
+
+import (
+	"code.gitea.io/gitea/modules/util"
+
+	"golang.org/x/net/html"
+)
+
+func visitNodeImg(ctx *RenderContext, img *html.Node) (next *html.Node) {
+	next = img.NextSibling
+	for i, attr := range img.Attr {
+		if attr.Key != "src" {
+			continue
+		}
+
+		if IsNonEmptyRelativePath(attr.Val) {
+			attr.Val = util.URLJoin(ctx.Links.ResolveMediaLink(ctx.IsWiki), attr.Val)
+
+			// By default, the "<img>" tag should also be clickable,
+			// because frontend use `<img>` to paste the re-scaled image into the markdown,
+			// so it must match the default markdown image behavior.
+			hasParentAnchor := false
+			for p := img.Parent; p != nil; p = p.Parent {
+				if hasParentAnchor = p.Type == html.ElementNode && p.Data == "a"; hasParentAnchor {
+					break
+				}
+			}
+			if !hasParentAnchor {
+				imgA := &html.Node{Type: html.ElementNode, Data: "a", Attr: []html.Attribute{
+					{Key: "href", Val: attr.Val},
+					{Key: "target", Val: "_blank"},
+				}}
+				parent := img.Parent
+				imgNext := img.NextSibling
+				parent.RemoveChild(img)
+				parent.InsertBefore(imgA, imgNext)
+				imgA.AppendChild(img)
+			}
+		}
+		attr.Val = camoHandleLink(attr.Val)
+		img.Attr[i] = attr
+	}
+	return next
+}
+
+func visitNodeVideo(ctx *RenderContext, node *html.Node) (next *html.Node) {
+	next = node.NextSibling
+	for i, attr := range node.Attr {
+		if attr.Key != "src" {
+			continue
+		}
+		if IsNonEmptyRelativePath(attr.Val) {
+			attr.Val = util.URLJoin(ctx.Links.ResolveMediaLink(ctx.IsWiki), attr.Val)
+		}
+		attr.Val = camoHandleLink(attr.Val)
+		node.Attr[i] = attr
+	}
+	return next
+}