Adding api-key and domains for prod/stage for mailgun

- Part of #530
- Only need DNS changes to complete
Merge pull request #570 from rackerlabs/gh-530-a

Adding api-key and domains for prod/stage for mailgun
Fixing missed image references in blog posts

- Fixes #573
Merge pull request #574 from rackerlabs/gh-573

Fixing missed image references in blog posts
Switching contact emails to be templated from md/text files
add text to sponsor email markdown template
update sponsor-email markdown template and add text to sponsor-email text template
Merge pull request #575 from rackerlabs/sponsor-email-redux

Update sponsorship email copy
Adding charitable 501c3 field

- fixes #542
Adding charitable event as a radio button
i r dum
Merge pull request #576 from rackerlabs/gh-542

Adding charitable 501c3 field
atom.xml feed for the blog
Merge pull request #578 from rackerlabs/atom-feed-redux

atom.xml feed for the blog
Setting sponsorships notification email to my personal one for staging.