Add popular-enabled option

This is similar to the removed `top-enabled` option but for the Popular feed.
The instance needs to be restarted if the feed was enabled.

Editing admin options on the preferences page is also fixed.

The handling of the feed pages now only happens in a single place.

Instead of redirecting:
  - The Top feed now displays a message that it was removed from Invidious.
  - The Popular feed now displays a message that it was disabled if it was.

src/invidious/views/message.ecr

@@ -0,0 +1,12 @@
+<% content_for "header" do %>
+<meta name="description" content="<%= translate(locale, "An alternative front-end to YouTube") %>">
+<title>
+    Invidious
+</title>
+<% end %>
+
+<%= rendered "components/feed_menu" %>
+
+<p>
+    <%= message %>
+</p>

src/invidious/views/preferences.ecr

@@ -227,6 +227,12 @@
                     <% end %>
                 </div>
 
+                <div class="pure-control-group">
+                    <label for="popular_enabled"><%= translate(locale, "Popular enabled: ") %></label>
+                    <input name="popular_enabled" id="popular_enabled" type="checkbox" <% if config.popular_enabled %>checked<% end %>>
+                </div>
+
+
                 <div class="pure-control-group">
                     <label for="captcha_enabled"><%= translate(locale, "CAPTCHA enabled: ") %></label>
                     <input name="captcha_enabled" id="captcha_enabled" type="checkbox" <% if config.captcha_enabled %>checked<% end %>>