Fix URLJoin, markup render link reoslving, sign-in/up/linkaccount page common data (#36861)

The logic of "URLJoin" is unclear and it is often abused.

Also:
* Correct the `resolveLinkRelative` behavior
* Fix missing "PathEscape" in `ToTag`
* Fix more FIXMEs, and add new FIXMEs for newly found problems
* Refactor "auth page common template data"

modules/setting/server.go

@@ -15,7 +15,6 @@ import (
 
 	"code.gitea.io/gitea/modules/json"
 	"code.gitea.io/gitea/modules/log"
-	"code.gitea.io/gitea/modules/util"
 )
 
 // Scheme describes protocol types
@@ -163,7 +162,7 @@ func MakeManifestData(appName, appURL, absoluteAssetURL string) []byte {
 }
 
 // MakeAbsoluteAssetURL returns the absolute asset url prefix without a trailing slash
-func MakeAbsoluteAssetURL(appURL, staticURLPrefix string) string {
+func MakeAbsoluteAssetURL(appURL *url.URL, staticURLPrefix string) string {
 	parsedPrefix, err := url.Parse(strings.TrimSuffix(staticURLPrefix, "/"))
 	if err != nil {
 		log.Fatal("Unable to parse STATIC_URL_PREFIX: %v", err)
@@ -171,11 +170,12 @@ func MakeAbsoluteAssetURL(appURL, staticURLPrefix string) string {
 
 	if err == nil && parsedPrefix.Hostname() == "" {
 		if staticURLPrefix == "" {
-			return strings.TrimSuffix(appURL, "/")
+			return strings.TrimSuffix(appURL.String(), "/")
 		}
 
 		// StaticURLPrefix is just a path
-		return util.URLJoin(appURL, strings.TrimSuffix(staticURLPrefix, "/"))
+		appHostURL := &url.URL{Scheme: appURL.Scheme, Host: appURL.Host}
+		return appHostURL.String() + "/" + strings.Trim(staticURLPrefix, "/")
 	}
 
 	return strings.TrimSuffix(staticURLPrefix, "/")
@@ -316,7 +316,7 @@ func loadServerFrom(rootCfg ConfigProvider) {
 		Domain = urlHostname
 	}
 
-	AbsoluteAssetURL = MakeAbsoluteAssetURL(AppURL, StaticURLPrefix)
+	AbsoluteAssetURL = MakeAbsoluteAssetURL(appURL, StaticURLPrefix)
 	AssetVersion = strings.ReplaceAll(AppVer, "+", "~") // make sure the version string is clear (no real escaping is needed)
 
 	manifestBytes := MakeManifestData(AppName, AppURL, AbsoluteAssetURL)