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/interfaces/security_price_provider_interface_test.rb

@@ -4,6 +4,7 @@ module SecurityPriceProviderInterfaceTest
   extend ActiveSupport::Testing::Declarative
 
   test "security price provider interface" do
+    assert_respond_to @subject, :healthy?
     assert_respond_to @subject, :fetch_security_prices
   end