log_in_user: extract user session setup from LoginController

The next changeset will need to set up a user login session outside
LoginController. 'log_in_user' extracted and added as a top-level
function in kallithea.lib.base since the code doesn't need access to
the current controller.

Code refactored to take a User object instead of a username, to allow
callers flexibility in how the user should be looked up.