[#817] Add email whitelist

This adds the ability to filter user signup with a whitelist
instead of or in addition to a blacklist.

Fixes #817

config/initializers/blacklists.rb

@@ -2,4 +2,5 @@
 
 Rails.application.configure do
   config.x.email_domains_blacklist = ENV.fetch('EMAIL_DOMAIN_BLACKLIST') { 'mvrht.com' }
+  config.x.email_domains_whitelist = ENV.fetch('EMAIL_DOMAIN_WHITELIST') { '' }  
 end