Restructure Gatsby-WordPress template to match other multiapps

[gilzow]

The WordPress part of the gatsby-wordpress multiapp uses a similar set up as our WordPress-Composer template, but not completely. For ease of maintenance, the suggestion is to bring the two :

• Move the copying of the wp-config.php file from a composer script to the build hook
• Add a step to the build hook to copy files/directories from a plugins directory to wordpress/wp-content/plugins to handle plugins that aren't available via composer

Also, update the root directory names to match what we have in the nextjs-wordpress/drupal template.

[gilzow]

Combine this issue with PR #612