users: add account upgrade integration tests.

* Test that the user upgrade process integrates with Stripe correctly.
* Replace a deprecated `card` param with `source` in `Stripe::Charge.create`.
* Rescue Stripe::StripeError instead of Stripe::CardError so that we
  handle failures outside of card failures, such as network errors.

Gemfile.lock

@@ -134,6 +134,7 @@ GEM
     concurrent-ruby (1.1.7)
     crass (1.0.6)
     daemons (1.3.1)
+    dante (0.2.0)
     delayed_job (4.1.8)
       activesupport (>= 3.0, < 6.1)
     delayed_job_active_record (4.1.4)
@@ -354,6 +355,10 @@ GEM
     streamio-ffmpeg (3.0.2)
       multi_json (~> 1.8)
     stripe (5.28.0)
+    stripe-ruby-mock (3.0.1)
+      dante (>= 0.2.0)
+      multi_json (~> 1.0)
+      stripe (> 5, < 6)
     thor (1.0.1)
     thread_safe (0.3.6)
     tzinfo (1.2.8)
@@ -452,6 +457,7 @@ DEPENDENCIES
   stackprof
   streamio-ffmpeg
   stripe
+  stripe-ruby-mock
   unicorn
   unicorn-worker-killer
   webpacker (>= 4.0.x)