Remove warp_theme and add option to exclude wp-content in wp_app

The vagrant development environment for warp themes will share/sync
the whole wordpress wp-content directory; therefore, wp_app will add
the option to install a wordpress site without wp-content and the
environment won't need a separate warp_theme resource. Also, stop
creating a plugins directory since each theme in the environment
handles the plugins directly through wp-content (in fact, warp_theme
was the only resource that used the plugins directory).