Add IP-based block list to wp-config

[matt-bernhardt]

Why are these changes being introduced:

We are starting to see problematic site traffic, and want to have a way to block access to the application selectively.

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/pw-86

How does this address that need:

This adds an ability to block specific IP addresses from getting site responses, sending a 403 status message instead. The list of blocked IP addresses is managed via environment variables.

Document any side effects to this change:

There is a small increase in site overhead, as every page load will now be checked against the IP block list. However, this is done at the PHP level, without needing to spin up WordPress itself.

Developer

Stylesheets

• Any theme or plugin whose stylesheets have changed has had its version
  string incremented.

Secrets

• All new secrets have been added to Pantheon tiers
• Relevant secrets have been updated in Github Actions
• All new secrets documented in README

Documentation

• Project documentation has been updated
• No documentation changes are needed

Accessibility

• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• The UI is not affected by this work.

Stakeholder approval

• Stakeholder approval has been confirmed
• Stakeholder approval is not needed

Dependencies

NO dependencies are updated

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• The changes have been verified
• The documentation has been updated or is unnecessary
• New dependencies are appropriate or there were no changes