Refactor user settings

Use resources instead of custom controllers, following the Rails way
and making things much cleaner in the process.

app/views/settings/show.html.erb

@@ -0,0 +1,5 @@
+<%= render HeaderComponent.new(title: "Settings") %>
+
+<%= render MainWithSidenavComponent.new(sidenav_partial: 'shared/sidenav_settings') do %>
+  <%= render partial: @settings_section %>
+<% end %>