Add "report abuse" prompts for users (disabled by default)

[0secure]

This change adds two new environment variables (ABUSE_ORG and ABUSE_EMAIL) that are used to direct users to contact a particular organization in case of suspected abuse. In addition, a generic 404 page and a secret-specific 404 page are added to minimize confusion and direct users to report abuse. If neither environment variable is set, the prompt to report abuse is not shown.

[jparise]

@0secure just wanted to follow up on adding some README.rst documentation for this new feature so we can merge it.

[jparise]

@0secure do you think you'll be picking this work back up?