Changed how the command registration was handled due to complications of
action order found during testing. Now the theme service will resolve
and directly register the command on the Kernel instead of them being
fetched from the ThemeService from within Kernel.
More direct, Seems to work.
System was using the empty state return from theme_path,
when no theme was configured, for loading in languages
which would result in the root path being looked up upon.
This changes the theme_path helper to return null in cases a theme
is not configured instead of empty string to help prevent assumed
return path will be legitimate, and to help enforce error case
handling.
For #2836
Allows someone using the theme system to configure the social driver
before a redirect action occurs, by passing a callback as an additional
param to the theme 'addSocialDriver' method.
