Basic donation records

Adds donation model/table and basic manual management in the admin
panel, as well as basic listing of users' own donations.

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

@@ -0,0 +1,39 @@
+<h2>Donations</h2>
+
+<% if @donations.any? %>
+  <table>
+    <thead>
+      <tr>
+        <th>User</th>
+        <th>Amount BTC (sats)</th>
+        <th>in EUR</th>
+        <th>in USD</th>
+        <th>Public name</th>
+        <th colspan="3"></th>
+      </tr>
+    </thead>
+
+    <tbody>
+      <% @donations.each do |donation| %>
+        <tr>
+          <td><%= donation.user.cn %></td>
+          <td><%= donation.amount_sats %></td>
+          <td><%= donation.amount_eur %></td>
+          <td><%= donation.amount_usd %></td>
+          <td><%= donation.public_name %></td>
+          <td><%= link_to 'Show', admin_donation_path(donation) %></td>
+          <td><%= link_to 'Edit', edit_admin_donation_path(donation) %></td>
+          <td><%= link_to 'Destroy', admin_donation_path(donation), method: :delete, data: { confirm: 'Are you sure?' } %></td>
+        </tr>
+      <% end %>
+    </tbody>
+  </table>
+<% else %>
+  <p>
+    No donations yet.
+  </p>
+<% end %>
+
+<p class="actions">
+  <%= link_to 'Record an out-of-system donation', new_admin_donation_path %>
+</p>