Makes mail method a single resource

Fixes #2643
No need for 'Closes #1234' in commit messages

I'm happy with the current workflow of pulling down, reviewing changes and amending as necessary.
Correct documentation for title helper

Fixes #2878
Only set ActiveMerchant::Billing::Base if gateway_options[:server] is available
Make StockLocation#stock_item return real first stock item

Fixes build breakage with PostgreSQL
Stub spree_api_key call in MailMethodsController

So that when this controller is called it doesn't attempt to generate one
Remove references to defunct MailMethod factory
Fix indeterminate ordering problem in stock_location_spec
Bump highline to 1.6.18
Show both name and sku of variant on transfer stock form
Allow users to not override default mailer configs

Fix from_address logic in mailers, it broke when there were noe
mail_method record on the database