Add LndHub db/models, and quick stats for admin views

app/views/admin/invitations/index.html.erb

@@ -2,16 +2,28 @@
 
 <%= render MainSimpleComponent.new do %>
   <section>
-    <p>
-      There are currently <strong><%= @invitations_unused_count %>
-      unused invitations</strong> available to existing users.
-      <strong><%= @users_with_referrals_count %> users</strong> have successfully
-      invited new users.
-    </p>
+    <%= render QuickstatsContainerComponent.new do %>
+      <%= render QuickstatsItemComponent.new(
+          type: :number,
+          title: 'Available',
+          value: @stats[:available],
+      ) %>
+      <%= render QuickstatsItemComponent.new(
+          type: :number,
+          title: 'Accepted',
+          value: @stats[:accepted],
+      ) %>
+      <%= render QuickstatsItemComponent.new(
+          type: :number,
+          title: 'Users with referrals',
+          value: @stats[:users_with_referrals],
+          meta: "/ #{User.count}"
+      ) %>
+    <% end %>
   </section>
   <% if @invitations_used.any? %>
     <section>
-      <h3>Accepted (<%= @invitations_used.length %>)</h3>
+      <h3>Recently Accepted</h3>
       <table>
         <thead>
           <tr>