Add session data to env vars

Currently the session secret can only be set by config.json or docker
secrets. This creates a problem on Heroku hosted instances that can not
set a session secret.

Since we automatically generate them on startup this results in an
logout of all users on every config change in Heroku.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

app.json

@@ -23,6 +23,10 @@
             "description": "Specify database type. See sequelize available databases. Default using postgres",
             "value": "postgres"
         },
+        "HMD_SESSION_SECRET": {
+            "description": "Secret used to secure session cookies.",
+            "required": false
+        },
         "HMD_HSTS_ENABLE": {
             "description": "whether to also use HSTS if HTTPS is enabled",
             "required": false