Improve consistency in naming

Summary

We're making naming more consistent:

Rename `Event#spree_payment_method` to `Event#payment_method`

We're using the prefix-less version for the Solidus payment method elsewhere.

Prefix factories with `:solidus_stripe` instead of `:stripe`

Having something like build(:stripe_payment_method) can be confusing because Stripe also has a different Payment Method entity. We're now renaming the factory for SolidusStripe::PaymentMethod instances to :solidus_stripe_payment_method, and using the same prefix for all other factories for consistency.

Rename `:order_with_stripe_payment` factory to `:solidus_stripe_order`

That's to be consistent with all the other factories, which are prefixed with solidus_stripe.

Prefix Stripe refund references with `:stripe_`

Prepend references to Stripe payment intents with `:stripe_`

Closes #283

Checklist

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Codecov Report

Merging #289 (0b070ec) into master (5e7a18a) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #289   +/-   ##
=======================================
  Coverage   99.62%   99.62%           
=======================================
  Files          30       30           
  Lines         540      540           
=======================================
  Hits          538      538           
  Misses          2        2

Impacted Files	Coverage Δ
app/models/solidus_stripe/gateway.rb	`100.00% <100.00%> (ø)`
...ribers/solidus_stripe/webhook/charge_subscriber.rb	`100.00% <100.00%> (ø)`
...olidus_stripe/webhook/payment_intent_subscriber.rb	`100.00% <100.00%> (ø)`
lib/solidus_stripe/refunds_synchronizer.rb	`100.00% <100.00%> (ø)`
lib/solidus_stripe/testing_support/factories.rb	`100.00% <100.00%> (ø)`
lib/solidus_stripe/webhook/event.rb	`96.96% <100.00%> (ø)`

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solidusio / solidus_stripe