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"
/>

routers/web/auth/oauth.go

@@ -541,6 +541,16 @@ func GrantApplicationOAuth(ctx *context.Context) {
 		ctx.Error(http.StatusBadRequest)
 		return
 	}
+
+	if !form.Granted {
+		handleAuthorizeError(ctx, AuthorizeError{
+			State:            form.State,
+			ErrorDescription: "the request is denied",
+			ErrorCode:        ErrorCodeAccessDenied,
+		}, form.RedirectURI)
+		return
+	}
+
 	app, err := auth.GetOAuth2ApplicationByClientID(ctx, form.ClientID)
 	if err != nil {
 		ctx.ServerError("GetOAuth2ApplicationByClientID", err)

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

templates/user/auth/grant.tmpl

@@ -23,8 +23,8 @@
 					<input type="hidden" name="scope" value="{{.Scope}}">
 					<input type="hidden" name="nonce" value="{{.Nonce}}">
 					<input type="hidden" name="redirect_uri" value="{{.RedirectURI}}">
-					<button type="submit" id="authorize-app" value="{{ctx.Locale.Tr "auth.authorize_application"}}" class="ui red inline button">{{ctx.Locale.Tr "auth.authorize_application"}}</button>
+					<button type="submit" id="authorize-app" name="granted" value="true" class="ui red inline button">{{ctx.Locale.Tr "auth.authorize_application"}}</button>
-					<a href="{{.RedirectURI}}" class="ui basic primary inline button">Cancel</a>
+					<button type="submit" name="granted" value="false" class="ui basic primary inline button">{{ctx.Locale.Tr "cancel"}}</button>
 				</form>
 			</div>
 		</div>