feat: remove prod dependency to socket.io-client

The client bundles are included in the repository in order to remove
socket.io-client from the list of production dependencies and thus to
reduce the total number of dependencies when installing the server.

This means the release of the client and the server must now be in sync
(which is almost always the case actually).

The minified build is now served:

- /<path>/socket.io.js
- /<path>/socket.io.js.map
- /<path>/socket.io.min.js
- /<path>/socket.io.min.js.map

The content will now be compressed as well.