Account Issue Model and Resolution Flow + Troubleshooting guides (#1090)

* Rough draft of issue system

* Simplify design

* Remove stale files from merge conflicts

* STI for issues

* Cleanup

* Improve Synth api key flow

* Stub api key for test

test/controllers/issue/exchange_rate_provider_missings_controller_test.rb

@@ -0,0 +1,19 @@
+require "test_helper"
+
+class Issue::ExchangeRateProviderMissingsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    sign_in users(:family_admin)
+    @issue = issues(:one)
+  end
+
+  test "should update issue" do
+    patch issue_exchange_rate_provider_missing_url(@issue), params: {
+      issue_exchange_rate_provider_missing: {
+        synth_api_key: "1234"
+      }
+    }
+
+    assert_enqueued_with job: AccountSyncJob
+    assert_redirected_to account_url(@issue.issuable)
+  end
+end