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/concerns/issuable.rb

@@ -0,0 +1,58 @@
+module Issuable
+  extend ActiveSupport::Concern
+
+  included do
+    has_many :issues, dependent: :destroy, as: :issuable
+  end
+
+  def has_issues?
+    issues.active.any?
+  end
+
+  def resolve_stale_issues
+    issues.active.each do |issue|
+      issue.resolve! if issue.stale?
+    end
+  end
+
+  def observe_unknown_issue(error)
+    observe_issue(
+      Issue::Unknown.new(data: { error: error.message })
+    )
+  end
+
+  def observe_missing_exchange_rates(from:, to:, dates:)
+    observe_issue(
+      Issue::ExchangeRatesMissing.new(data: { from_currency: from, to_currency: to, dates: dates })
+    )
+  end
+
+  def observe_missing_exchange_rate_provider
+    observe_issue(
+      Issue::ExchangeRateProviderMissing.new
+    )
+  end
+
+  def observe_missing_price(ticker:, date:)
+    issue = issues.find_or_create_by(type: Issue::PricesMissing.name, resolved_at: nil)
+    issue.append_missing_price(ticker, date)
+    issue.save!
+  end
+
+  def highest_priority_issue
+    issues.active.ordered.first
+  end
+
+  private
+
+    def observe_issue(new_issue)
+      existing_issue = issues.find_by(type: new_issue.type, resolved_at: nil)
+
+      if existing_issue
+        existing_issue.update!(last_observed_at: Time.current, data: new_issue.data)
+      else
+        new_issue.issuable = self
+        new_issue.save!
+      end
+    end
+end