Custom Block Status Code

[rswalkden]

• Allows a custom block status code to be applied, controlled by the NETACEA_BLOCKED_RESPONSE_STATUS environment variable
• Optionally, a custom body may be set too, using NETACEA_BLOCKED_RESPONSE_BODY. When a custom body is set it is advisable to also explicitly set the content type using NETACEA_BLOCKED_RESPONSE_CONTENT_TYPE.
• Included in the PR are two bug fixes:
  • Read large captcha bodies that have been spilled to disk. Previously large bodies would fail to send.
  • Set the correct bc_type for the captcha-on-demand path. This was a minor logging issue, but the fix makes it clearer in our logs what action has been applied.