Users session

This PR add users sessions via the [node-client-sessions](https://github.com/mozilla/node-client-sessions) module.

For now, there is no token generated. The email is still used to identify someone but this time no one should be able to impersonate someone else as they have not access to the cookie.

It simplify the `Server` abstraction present in the talkilla SPA, thus there is a few modifications on this side.

Check the Trello card for more info: https://trello.com/c/coNdWC11