Add WP_DEBUG options to the wordpress app parameters

The original implementation was using a define constant, which meant
that the additional configuration file could not override the pre-set
values from the initial template.

This commit adds:

- WP_DEBUG
- WP_DEBUG_LOG
- WP_DEBUG_DISPLAY

All values default to false, which matches the initial implementation
for WP_DEBUG and continues that intent for the new values.
Could you remove the pkg directory from the commit? Something like `git rm -rf pkg && git commit --amend` and force push.