Install redash-stmo.

In the long run we'll be able to install additional dependencies by
having an own Dockerfile to build images based on the Redash image
but that installs additional Python dependencies. But until we have
a fork with lots of changes ourselves we need to do it this way.

Redash-stmo contains the ability to hook up our own Dockerflow
library.

Refs #13
Refs #37

requirements.txt

@@ -61,3 +61,4 @@ disposable-email-domains
 # It is not included by default because of the GPL license conflict.
 # ldap3==2.2.4
 gevent==1.4.0
+redash-stmo>=2018.4.0