Set site-wide 'from' address

Config/initializers/devise.rb sets this for devise, but not for mailers.
https://www.pivotaltracker.com/story/show/41278297
Email volunteers when they volunteer

https://www.pivotaltracker.com/s/projects/608983
Merge branch 'master' of https://github.com/railsbridge/bridge_troll into rsvp-confirmation-mailer
Merge branch 'rsvp-confirmation-mailer' of https://github.com/k-stewart/bridge_troll into k-stewart-rsvp-confirmation-mailer