Rework markup link rendering (#26745) (#28803)

Backport #26745
Fixes #26548

This PR refactors the rendering of markup links. The old code uses
`strings.Replace` to change some urls while the new code uses more
context to decide which link should be generated.

The added tests should ensure the same output for the old and new
behaviour (besides the bug).

We may need to refactor the rendering a bit more to make it clear how
the different helper methods render the input string. There are lots of
options (resolve links / images / mentions / git hashes / emojis / ...)
but you don't really know what helper uses which options. For example,
we currently support images in the user description which should not be
allowed I think:

<details>
  <summary>Profile</summary>

https://try.gitea.io/KN4CK3R

![grafik](109ae422-496d-4200-b52e-b3a528f553e5)

</details>

(cherry picked from commit 022552d5b6)

modules/markup/renderer.go

@@ -16,6 +16,7 @@ import (
 
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/util"
 
 	"github.com/yuin/goldmark/ast"
 )
@@ -69,7 +70,7 @@ type RenderContext struct {
 	RelativePath     string // relative path from tree root of the branch
 	Type             string
 	IsWiki           bool
-	URLPrefix        string
+	Links            Links
 	Metas            map[string]string
 	DefaultLink      string
 	GitRepo          *git.Repository
@@ -80,6 +81,45 @@ type RenderContext struct {
 	InStandalonePage bool // used by external render. the router "/org/repo/render/..." will output the rendered content in a standalone page
 }
 
+type Links struct {
+	Base       string
+	BranchPath string
+	TreePath   string
+}
+
+func (l *Links) HasBranchInfo() bool {
+	return l.BranchPath != ""
+}
+
+func (l *Links) SrcLink() string {
+	return util.URLJoin(l.Base, "src", l.BranchPath, l.TreePath)
+}
+
+func (l *Links) MediaLink() string {
+	return util.URLJoin(l.Base, "media", l.BranchPath, l.TreePath)
+}
+
+func (l *Links) RawLink() string {
+	return util.URLJoin(l.Base, "raw", l.BranchPath, l.TreePath)
+}
+
+func (l *Links) WikiLink() string {
+	return util.URLJoin(l.Base, "wiki")
+}
+
+func (l *Links) WikiRawLink() string {
+	return util.URLJoin(l.Base, "wiki/raw")
+}
+
+func (l *Links) ResolveMediaLink(isWiki bool) string {
+	if isWiki {
+		return l.WikiRawLink()
+	} else if l.HasBranchInfo() {
+		return l.MediaLink()
+	}
+	return l.Base
+}
+
 // Cancel runs any cleanup functions that have been registered for this Ctx
 func (ctx *RenderContext) Cancel() {
 	if ctx == nil {