Refactor admin settings routes

This is much cleaner, and semantically more correct.

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

@@ -0,0 +1,32 @@
+<%= render HeaderComponent.new(title: "Settings") %>
+
+<%= render MainWithSidenavComponent.new(sidenav_partial: 'shared/admin_sidenav_settings') do %>
+  <%= form_for(Setting.new, url: admin_settings_registrations_path, method: :put) do |f| %>
+    <section>
+      <h3>Registrations</h3>
+
+      <% if @errors && @errors.any? %>
+        <%= render partial: "admin/settings/errors", locals: { errors: @errors } %>
+      <% end %>
+
+      <label class="block">
+        <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>
+    </section>
+
+    <section>
+      <p class="pt-6 border-t border-gray-200 text-right">
+        <%= f.submit 'Save', class: "btn-md btn-blue w-full md:w-auto" %>
+      </p>
+    </section>
+  <% end %>
+<% end %>