UX: warn that something must be selected with safe mode

app/controllers/safe_mode_controller.rb

@@ -14,6 +14,7 @@ class SafeModeController < ApplicationController
     if safe_mode.length > 0
       redirect_to path("/?safe_mode=#{safe_mode.join("%2C")}")
     else
+      flash[:must_select] = true
       redirect_to safe_mode_path
     end
   end

app/views/safe_mode/index.html.erb

@@ -6,24 +6,29 @@
     </p>
     <p>
     <label>
-    <%= check_box_tag 'no_customizations', true, true %>
+    <%= check_box_tag 'no_customizations', true, !flash[:must_select]%>
     <%= t 'safe_mode.no_customizations' %>
     </label>
     </p>
     <p>
     <label>
-    <%= check_box_tag 'only_official', true, true %>
+    <%= check_box_tag 'only_official', true, !flash[:must_select] %>
     <%= t 'safe_mode.only_official' %>
     </label>
     </p>
     <p>
     <label>
-    <%= check_box_tag 'no_plugins', true, true %>
+    <%= check_box_tag 'no_plugins', true, !flash[:must_select] %>
     <%= t 'safe_mode.no_plugins' %>
     </label>
     </p>
     <p>
       <%= submit_tag t('safe_mode.enter'), class: 'btn btn-danger' %>
+      <%- if flash[:must_select] %>
+        <span class='safe-mode-warning'>
+          <i class='fa fa-warning'></i> <%= t 'safe_mode.must_select' %>
+        </span>
+      <%- end %>
     </p>
   <% end %>
 </div>

config/locales/server.en.yml

@@ -3503,6 +3503,7 @@ en:
     only_official: "Disable unofficial plugins"
     no_plugins: "Disable all plugins"
     enter: "Enter Safe Mode"
+    must_select: "You must select at least one option to enter safe mode."
   wizard:
     title: "Discourse Setup"
     step: