Slim down mailer payloads by including less CSS

[jagthedrummer]

Including both application.css and application.mailer.light.css is redundant, and application.css includes a lot of things that aren't needed in mailers.

If your mailers are depending on something in application.css you can bring it back by first ejecting the _mailer.html.erb partial into your application:

./bin/resolve bullet_train-themes-light/app/views/themes/light/layouts/_mailer.html.erb --eject

Then look for this line:

<%= stylesheet_link_tag 'application.mailer.light', media: 'all' %>

And add a link to the application stylesheet above it like this:

<%= stylesheet_link_tag 'application', media: 'all' %>
<%= stylesheet_link_tag 'application.mailer.light', media: 'all' %>

Fixes #293