Paper cache + reload button

[bzz]

Right now the backend is completely stateless: every request from the frontend triggers fetching from GMail.

The idea is

• to cache papers per user session in-memory on the backend and serve only them from /json/messages
• add explicit "Reload" action to the frontend that will trigger a new /json/messages/fetch

This will allow to experiment on real data much faster and will be a fist step towards introducing a proper state management on the backend.