check if signups are allowed during SignUpPost

Previously you could hide the sign up page with the following
configs:

```
[service]
SHOW_REGISTRATION_BUTTON          = false
```

This would remove the sign up button from the navbar, but the user
could still access the page with a direct link. During the http post
action, the go action checked if the key `SHOW_REGISTRATION_BUTTON` was
set to true. If not, the user received a 403.

You can use this scenario for a semi-hidden sign up page that is only
accessible through a direct link.

I've changed the check in this PR to check if the sign up is allowed
or not. The user can access the sign up page (if it's enabled) and
sign up even if the value of `SHOW_REGISTRATION_BUTTON` is `false`, the
user can sign up.

Another solution could be:
- unify `SHOW_REGISTRATION_BUTTON ` with `DISABLE_REGISTRATION ` to
only show the registration page if it's enabled, but that would dis-
allow the mentioned scenario

fixes: go-gitea#5183
Signed-off-by: Roman <romaaan.git@gmail.com>

routers/user/auth.go

@@ -923,7 +923,7 @@ func SignUpPost(ctx *context.Context, cpt *captcha.Captcha, form auth.RegisterFo
 	ctx.Data["RecaptchaSitekey"] = setting.Service.RecaptchaSitekey
 
 	//Permission denied if DisableRegistration or AllowOnlyExternalRegistration options are true
-	if !setting.Service.ShowRegistrationButton {
+	if setting.Service.DisableRegistration || setting.Service.AllowOnlyExternalRegistration {
 		ctx.Error(403)
 		return
 	}