Remove wp-app directory and replace with WP CLI commands

[jeffcullis]

The full plugin downloads are currently stored within the repo at:
cioos-siooc-wordpress/wp-app/wp-content/plugins
However these can quickly become out of date so we will instead we will add this directory to the .gitignore.

To make it easier to install required plugins on initial deployment, we can document the plugins and provide links to them. Another option might be to create a wp-cli command that would enable installation of all plugins for a particular RA with a single command.