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

app/models/issue/unknown.rb

@@ -0,0 +1,11 @@
+class Issue::Unknown < Issue
+  def default_severity
+    :warning
+  end
+
+  # Unknown issues are always stale because we only want to show them
+  # to the user once.  If the same error occurs again, we'll create a new instance.
+  def stale?
+    true
+  end
+end