Don't hardcode /socket.io

This small PR allows to run Etherpad on a base URL. For example, we run multiple etherpad servers and shard them through an identifier in the URL.

/etherpad/0/* ----> go to machine 0
/etherpad/1/*  ---> go to machine 1
etc..

This fix allows us to do that as now we have URLs such as /etherpad/0/socket.io, /etherpad/1/socket.io ,etc..