Skeleton for remote query execution using celery. (#908)

* Carapal react mockup

This is really just a mock up written in React to try different
components. It could become scaffolding to build a prototype, or not.

* Preliminary commit for Celery backend

* Move the SQL query execution to the celery worker.

* React scetch

* Refactor SQL execution to use the celery if configured.

* Refactor SQL execution to use the celery if configured.

* Add query model

* Remove QueryResult. Query has a tmp_table_name field that has all the data.

* Add create table as wrapper.

* Create table as

* Address the comments.

* Add trailing commas

* Remove the init_query test.

* Handle 'undefined' schema case

Author: bkyryliuk

caravel/views.py

@@ -1348,7 +1348,6 @@ def sql_json(self):
         sql = request.form.get('sql')
         database_id = request.form.get('database_id')
         schema = request.form.get('schema')
-        mydb = session.query(models.Database).filter_by(id=database_id).first()
 
         if not (self.can_access(
                 'all_datasource_access', 'all_datasource_access') or