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.

example.env.py

@@ -0,0 +1,11 @@
+# Please see example.env for a detailed list of supported environment
+# variables as well as their semantics.
+
+# NOTE: this file is not meant for production use. Due to the non-constant
+# secret key, each server restart will log out all users from the web portal.
+
+import secrets
+SNIKKET_WEB_SECRET_KEY = secrets.token_urlsafe(nbytes=32)
+SNIKKET_WEB_PROSODY_ENDPOINT = "http://localhost:5280"
+SNIKKET_WEB_DOMAIN = "localhost"
+# SNIKKET_WEB_AVATAR_CACHE_TTL = 1800