First run through of updating styles to our guide

Added some BEM and UpperCamelCase to Timber - finally.

@carolineschnapp are address and password fields allowed to change the ID? I.e. from `password_confirmation` to `PasswordConfirmation` or `address_first_name_new` to `AddressFirstNameNew`? Can't find a reference to them in core, so my thought is it's safe to do.

cc/ @Shopify/fed