Merge commit '4ad1c5aa71' into glitch-soc/merge-upstream

Conflicts:
- `README.md`:
  Upstream has updated their README, we have a completely different one.
  Kept ours.
- `config/initializers/content_security_policy.rb`:
  Upstream has updated their development CSPs, while we disable CSPs in
  the development environment.
  Kept our version.

app/controllers/concerns/signature_verification.rb

@@ -119,6 +119,8 @@ module SignatureVerification
   private
 
   def fail_with!(message, **options)
+    Rails.logger.warn { "Signature verification failed: #{message}" }
+
     @signature_verification_failure_reason = { error: message }.merge(options)
     @signed_request_actor = nil
   end