Support connecting to postgres over UNIX socket

Currently, kresus requires that a connection to postgres happens on user:password@hostname:port.

But a better and often use setup is to use UNIX sockets instead.

This first need to be supported by typeorm as indicated by @nicofrand, but there is an existing PR: https://github.com/typeorm/typeorm/pull/6042

Then, kresus needs to support this case in its configuration file.