Convert to server-side sessions

[dmitrybelyakov]

Default flask sessions are client-side, where session data is encoded and stored on the client. We need to convert that to server side. This will allow more flexibility in app space as well as open possibilities for socket.io integration.

This potentially will require authentication refactoring.

Have a look at session data decoding: https://www.youtube.com/watch?v=mhcnBTDLxCI