Use seasocks lib to serve the websockets

[ThibaultNocchi]

Right now, two libs are used on the communication part of the server:

• websocketpp, used to expose websockets to the web client
• seasocks, used to run a simple file server to download files and corpus

websocketpp was found and used way before the need to serve static files came. Once we needed to download archives from the web client, we found seasocks to be really simple to build and use. But it also provides a websocket interface, which can be used on the same port as the HTTP server.

This change could free a lot of resources and an exposed port used by the server. But before doing so, one must check the viability of the seasocks library on the websockets side.