Clarify how intercom-rails resolves the current user

[shab2k03]

Summary

Use current_user as the default source for the Messenger user, and exclude unsaved user and company records from Messenger settings. Update the setup guidance and add coverage for both behaviours.

Behaviour change

The gem no longer reads @user automatically. In some controllers, @user is the record being displayed rather than the signed-in user, so it is not a reliable default. Applications that use @user for the signed-in user can continue to do so by configuring it explicitly:

config.user.current = Proc.new { @user }

Records that report themselves as unsaved are now excluded consistently, whether they expose new_record?, persisted?, or new?. Applications that already provide a saved current_user need no configuration change.