mailgun emails on 500, add/remove/list fire list emails

**Summary**: If greenkahuna/scrapbin.com return 500 errors, it will email the "on fire list" about it

Pivotal story: [will should email personal emails or text on 500 errors](https://www.pivotaltracker.com/story/show/65911870)


Staged at (if necessary): 

## Planning meeting
- [ ] I had a planning meeting with ____



## Pre-CR Checklist:
- [x] I have merged in master
- [x] I have done my own CR
- [x] I have played with the functionality a lot, am confident it works, and feels good for users.
- [x] The code is readable and understandable
- [ ] All functions have unit tests
- [ ] All functionality has e2e tests
- [ ] Tests pass
- [x] All points in the pivotal story are met, in letter and spirit
- [x] I am proud of what I made.

## CR 1

Checklist:
- [x] CR claimed by @skoczen
- [x] I have reviewed the entire code, and know what it does
- [x] I have placed all issues in inline comments, or the checklist below.

Fixes
- [x] Needs to require `will >=0.4.0`, then good to go!

## Good to merge!
- [x] PR good to merge by @skoczen