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.
If this is `define('WP_DEBUG_LOG', false);` will that break wordpress? If so, you'll have to surround this with `<% if @wp_debug_log -%>` lines. Ditto for `WP_DEBUG_DISPLAY`.