Guarantee payment is linked to payment intent when receiving webhooks

[waiting-for-dev]

Summary

We create and confirm the Stripe payment intent in two steps. That's to guarantee that we associate a Solidus payment on the creation, and we can fetch it after webhook events are published on confirmation.

Closes #231

Checklist

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[codecov[bot]]

Codecov Report

Merging #236 (f827aec) into master (7201766) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #236      +/-   ##
==========================================
+ Coverage   96.37%   96.39%   +0.02%     
==========================================
  Files          23       23              
  Lines         441      444       +3     
==========================================
+ Hits          425      428       +3     
  Misses         16       16              

Impacted Files	Coverage Δ
app/models/solidus_stripe/gateway.rb	96.49% <100.00%> (+0.19%)	:arrow_up:

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