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/templates/helper.go

@@ -37,7 +37,6 @@ func NewFuncMap() template.FuncMap {
 		"QueryEscape":  queryEscape,
 		"QueryBuild":   QueryBuild,
 		"SanitizeHTML": SanitizeHTML,
-		"URLJoin":      util.URLJoin,
 		"DotEscape":    dotEscape,
 
 		"PathEscape":         url.PathEscape,