Configure extra allowed params per endpoint via middleware

Instead of making all params available in all endpoints, we control
what endpoints allow what extra params.

Dataviews endpoints should be migrated to this.

lib/cartodb/middleware/allow-query-params.js

@@ -0,0 +1,6 @@
+module.exports = function allowQueryParams(params) {
+    return function allowQueryParamsMiddleware(req, res, next) {
+        req.context.allowedQueryParams = params;
+        next();
+    };
+};