enable houndci for PR's
add link to new s3 documentation in the heroku deployment guide

Closes #5641
Introduce DisplayMoney class mixin to clean up display_* method model logic.

There are a lot of model-level methods named display_ that just wrap a
numeric in a Spree::Money object. Rather than taking up space and mental
overhead in the models, this class mixin allows for defining of those
desired display_methods in a tidier way (with some configuration), in an effort to make some of
these models smaller and easier to reason about.

Closes #5640
Validate uniqueness of OptionType#name. Fixes #5629
Validate uniqueness of OptionValue#name. Fixes #5630
Sequence factories for uniqueness. refs #5629
Bump Rails.
reduce queries when running spree/stock specs

those should run over 40s faster now
Namespace eligibility validator like the rest.