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/views/issue/exchange_rate_provider_missings/show.html.erb

@@ -0,0 +1,28 @@
+<%= content_for :title, @issue.title %>
+
+<%= content_for :description do %>
+  <p>You have set your family currency preference to <%= Current.family.currency %>.  <%= @issue.issuable.name %> has
+    entries in another currency, which means we have to fetch exchange rates from a data provider to accurately show
+    historical results.</p>
+
+  <p>We have detected that your exchange rates provider is not configured yet.</p>
+<% end %>
+
+<%= content_for :action do %>
+  <% if self_hosted? %>
+    <p>To fix this issue, you need to provide an API key for your exchange rate provider.</p>
+
+    <p>Currently, we support <%= link_to "Synth Finance", "https://synthfinance.com", target: "_blank" %>, so you need
+      to
+      get a free API key from the link provided.</p>
+
+    <p>Once you have your API key, paste it below to configure it.</p>
+
+    <%= styled_form_with model: @issue, url: issue_exchange_rate_provider_missing_path(@issue), method: :patch, class: "space-y-3" do |form| %>
+      <%= form.text_field :synth_api_key, label: "Synth API Key", placeholder: "Synth API Key", type: "password", class: "w-full", value: Setting.synth_api_key %>
+      <%= form.submit "Save and Re-Sync Account", class: "btn-primary" %>
+    <% end %>
+  <% else %>
+    <p>Please contact the Maybe team.</p>
+  <% end %>
+<% end %>