Automatically generate a session secret if default is used

The session secret is used to sign and authenticate the session cookie
and this way very important for the authentication process.

By default the session secret is set to `secret` and never changes. This
commit will add a generator for a dynamic session secret if it stays
unchanged.

It prevents session hijacking this way and will warn the user about
the missing secret.

This also implies that on a restart without configured session secret
will log out all users. While it may seems annoying, it's for the users
best.

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

lib/config/default.js

@@ -46,6 +46,7 @@ module.exports = {
   // session
   sessionName: 'connect.sid',
   sessionSecret: 'secret',
+  sessionSecretLen: 128,
   sessionLife: 14 * 24 * 60 * 60 * 1000, // 14 days
   staticCacheTime: 1 * 24 * 60 * 60 * 1000, // 1 day
   // socket.io