More improvements to SQL Lab (#1104)

* Handling timeouts

* Fixing timer on non-utc server

* Allowing async with results

* [bugfix] database is not selected

* Making sure the session is up and running

* Cleaning up query results and query objects

* Picking a groupby and metric field on visualize flow

* Showing local time in query history

* Using pull-left pull-right instead of grid layout for table metdata

Long column name were looking weird and icons were wrapping oddly

* Linting

* Eliminating east buttons under the sql editor

* Sort database dropdown by name

* Linting

* Allowing non-SELECT statements to run

* Adding a db config

* Making sqla checkout check cross-db

docs/installation.rst

@@ -294,6 +294,25 @@ Upgrading should be as straightforward as running::
     pip install caravel --upgrade
     caravel db upgrade
 
+SQL Lab
+-------
+SQL Lab is a powerful SQL IDE that works with all SQLAlchemy compatible
+databases out there. By default, queries are run in a web request, and
+may eventually timeout as queries exceed the maximum duration of a web
+request in your environment, whether it'd be a reverse proxy or the Caravel
+server itself.
+
+In the modern analytics world, it's not uncommon to run large queries that
+run for minutes or hours.
+To enable support for long running queries that
+execute beyond the typical web request's timeout (30-60 seconds), it is
+necessary to deploy an asynchronous backend, which consist of one or many
+Caravel worker, which is implemented as a Celery worker, and a Celery
+broker for which we recommend using Redis or RabbitMQ.
+
+It's also preferable to setup an async result backend as a key value store
+that can hold the long-running query results for a period of time. More
+details to come as to how to set this up here soon.
 
 Making your own build
 ---------------------