app.rb: remove slash from URL in /auth-begin

REQUEST_URI already has a trailing slash so we shouldn't be adding a
second one.
FancyBear::ContextIO::Auth: move initializer to Base class

I need a ContextIO instance in another module class so let's create a
base class so I don't have to keep doing it.
Get label creation working by shelling out to python

Dammit Context.IO Ruby SDK
Merge branch 'feature/contextio-messages' into develop