Makes mail method a single resource

Related to #2643
Moves preferences from MailMethod to AppConfiguration

Related to #2643
Uses Spree::Config in MailSettings and MailInterceptor

Related to #2643
Changes mail method form to use Spree::Config

  - Deletes new and index templates from mail methods
  - Removes mail_method model spec. It adds value to the test suite
  - Fixes requests and controllers specs related to mail method
    refactoring

Related to #2643
Adapts testmail action to new mail settings style

Fixes #2643
Fixes mail settings and interceptor specs

Moves them to /core/spec/lib since both MailInterceptor and MailSettings
are classes from the /core package

Fixes #2643
Remove all remaining references to Spree::MailMethod

And the class itself. I believe the spree_mail_methods table should stay
for now so that can core team can talk about a strategy to advise users
the best way to upgrade and docs are updated

Fixes #2643