Refactor markup render system (#32612)

This PR removes (almost) all path tricks, and introduces "renderhelper"
package.

Now we can clearly see the rendering behaviors for comment/file/wiki,
more details are in "renderhelper" tests.

Fix #31411 , fix #18592, fix #25632 and maybe more problems. (ps: fix
#32608 by the way)

modules/markup/render_link.go

@@ -0,0 +1,42 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markup
+
+import (
+	"context"
+	"strings"
+
+	"code.gitea.io/gitea/modules/httplib"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/util"
+)
+
+func resolveLinkRelative(ctx context.Context, base, cur, link string, absolute bool) (finalLink string) {
+	if IsFullURLString(link) {
+		return link
+	}
+	if strings.HasPrefix(link, "/") {
+		if strings.HasPrefix(link, base) && strings.Count(base, "/") >= 4 {
+			// a trick to tolerate that some users were using absolut paths (the old gitea's behavior)
+			finalLink = link
+		} else {
+			finalLink = util.URLJoin(base, "./", link)
+		}
+	} else {
+		finalLink = util.URLJoin(base, "./", cur, link)
+	}
+	finalLink = strings.TrimSuffix(finalLink, "/")
+	if absolute {
+		finalLink = httplib.MakeAbsoluteURL(ctx, finalLink)
+	}
+	return finalLink
+}
+
+func (ctx *RenderContext) ResolveLinkRelative(base, cur, link string) (finalLink string) {
+	return resolveLinkRelative(ctx, base, cur, link, ctx.RenderOptions.UseAbsoluteLink)
+}
+
+func (ctx *RenderContext) ResolveLinkApp(link string) string {
+	return ctx.ResolveLinkRelative(setting.AppSubURL+"/", "", link)
+}