Support for Custom URI Schemes in OAuth2 Redirect URIs (#37356)

Fix #34349

By the way, remove `(ctx *APIContext) HasAPIError() ` and `(ctx
*APIContext) GetErrMsg()` because they do nothing, the error handling
has been done in API's middeware

The existing OAuth2 tests were not quite right, refactored them together

routers/web/shared/label/label.go

@@ -13,7 +13,7 @@ import (
 func GetLabelEditForm(ctx *context.Context) *forms.CreateLabelForm {
 	form := web.GetForm(ctx).(*forms.CreateLabelForm)
 	if ctx.HasError() {
-		ctx.JSONError(ctx.Data["ErrorMsg"].(string))
+		ctx.JSONError(ctx.GetErrMsg())
 		return nil
 	}
 	var err error