Fix login redirection links

[qwerty287]

Doesn't add the redirect_to argument if page is login.

fixes #17421 fixes #17444

[lunny]

And if the login router could check if the redirect_to is itself is better.

[FlyingOnion]

Consider href="{{if not .PageIsSignIn}}{{AppSubUrl}}/user/login?redirect_to={{end}}{{.CurrentURL}}" to keep the original link?

[wxiaoguang]

It sounds very cool, but it doesn't work:

package main

import (
	"html/template"
	"log"
	"os"
)

func main() {
	tplStr := `
	<a href="{{.url}}">
	<a href="?redirect={{.url}}">
	<a href="{{if true}}?{{end}}redirect={{.url}}">
	<a href="{{if not true}}?{{end}}redirect={{.url}}">
`
	tpl := template.Must(template.New("tplName").Parse(tplStr))
	err := tpl.Execute(os.Stdout, map[string]interface{}{"url":"/p?a=1&b=2#h", "true": true})
	log.Printf("err = %v", err)
}

error: 2021/10/28 17:23:38 err = html/template:tplName:4:40: {{.url}} appears in an ambiguous context within a URL

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@3fc465b). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main   #17451   +/-   ##
=======================================
  Coverage        ?   45.48%           
=======================================
  Files           ?      791           
  Lines           ?    88697           
  Branches        ?        0           
=======================================
  Hits            ?    40341           
  Misses          ?    41840           
  Partials        ?     6516           

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