WIP Add admin settings pages, reserved username config

Prototyping settings forms

app/views/admin/settings/registrations/index.html.erb

@@ -0,0 +1,31 @@
+<%= render HeaderComponent.new(title: "Settings") %>
+
+<%= render MainWithSidenavComponent.new(sidenav_partial: 'shared/admin_sidenav_settings') do %>
+  <section>
+    <h3>Registrations</h3>
+    <%= form_for(Setting.new, url: admin_settings_services_path) do |f| %>
+      <% if @errors && @errors.any? %>
+        <div>
+          <ul>
+            <% @errors.full_messages.each do |msg| %>
+              <li><%= msg %></li>
+            <% end %>
+          </ul>
+        </div>
+      <% end %>
+
+      <label>
+        <p class="font-bold mb-1">Reserved usernames</p>
+        <p class="text-gray-500">
+          These usernames cannot be registered as accounts:
+        </p>
+        <%= f.text_area :reserved_usernames,
+          value: Setting.reserved_usernames.join("\n"),
+          class: "h-44 mb-2" %>
+        <p class="text-sm text-gray-500">
+          One username per line
+        </p>
+      </label>
+    <% end %>
+  </section>
+<% end %>