Return access_denied error when an OAuth2 request is denied (#30974)

According to [RFC
6749](https://datatracker.ietf.org/doc/html/rfc6749#section-4.1.2.1),
when the resource owner or authorization server denied an request, an
`access_denied` error should be returned. But currently in this case
Gitea does not return any error.

For example, if the user clicks "Cancel" here, an `access_denied` error
should be returned.

<img width="360px"
src="https://github.com/go-gitea/gitea/assets/15528715/be31c09b-4c0a-4701-b7a4-f54b8fe3a6c5"
/>

services/forms/user_form.go

@@ -161,6 +161,7 @@ func (f *AuthorizationForm) Validate(req *http.Request, errs binding.Errors) bin
 // GrantApplicationForm form for authorizing oauth2 clients
 type GrantApplicationForm struct {
 	ClientID    string `binding:"Required"`
+	Granted     bool
 	RedirectURI string
 	State       string
 	Scope       string