Allow projects to be blacklisted for tips.

Allow authors to submit PRs to include their projects in the blacklist.yml to prevent them from accepting tips. By using a YAML file checked into Git we can propagate the blacklist to forks as well.

- [x] Prevent new projects from being created from blacklist.
- [x] Prevent tips from being given to existing projects that have since been blacklisted. 

More work needs to be done to provide a cleanup routine for existing projects that have since been blacklisted to return funds, mark project as disabled, etc. Someone with more domain knowledge should pick this up.

Ultimately this should be an opt-in system only, but that will require more time than I have to give. I hope this is a step in the right direction.

Fixes: #127, #145, #149, #150, #153, #155, #158, #159, #161, #162, #164, #166
Related: #146, #154, #157