12-factorize application a little

snikket_web can now be fully configured via the environment alone,
no extra files needed. It is still supported to inject a python
file to generate environment variables though, which may be
useful for generating and reloading a secret key.

requirements.txt

@@ -4,4 +4,5 @@ flask-wtf~=0.14
 hsluv~=0.0.2
 flask-babel~=1.0
 email-validator~=1.1
+environ-config~=20.0
 typing-extensions