added support for mailgun

**Summary**: Adds support for mailgun

Pivotal story: [will should email personal emails or text on 500 errors](https://www.pivotaltracker.com/story/show/65911870)


## Planning meeting
- [ ] I had a planning meeting with ________, or it was a small enough story it wasn't needed.


## 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:
- [ ] CR claimed by ______
- [ ] I have reviewed the entire code, and know what it does
- [ ] I have placed all issues in inline comments, or the checklist below.

CR Comment Review
- [ ] Comment review claimed by ________
- [ ] +1 / -1 (with reason) on all comments is done.

Fixes
- [ ] All comments to the story have been resolved
- [ ] Other comments go here

## FR 1

Checklist:
- [ ] FR claimed by ______
- [ ] I have used all the features of the story, and the software looks good and feels good.
- [ ] I have tested the features in Chrome
- [ ] I have tested the features in Firefox
- [ ] I have tested the features in Internet Explorer
- [ ] I have tested the features on an iPad
- [ ] I have tested the features on an Android Tablet
- [ ] I have tested the features on a Phone
- [ ] I have placed any issues in the FR checklist below

Fixes:
- [ ] Other comments go here

## Good to merge!
- [x] PR good to merge by @skoczen