Traces#index: Introduce tab navigation, fix tag-filter

- introduce bootstrap tabs to switch the views
- introduce `content_for :heading_class` to remove the padding below the bootstrap tabs
- update the rss-image to use a svg, adopted from https://icons.getbootstrap.com/icons/rss/ (without the outer border)
- move rss- and new-button away from the view-switching actions
- the `@tag` logic was broken. introduce new link to remove the tag-filter; the tabs keep the filter once given; use params[:tag] directly in the view
- use `&.` syntax nil-safety so we can remove `@display_name`

app/views/layouts/_content.html.erb

@@ -5,7 +5,7 @@
     <%= render :partial => "layouts/flash", :locals => { :flash => flash } %>
     <% if content_for? :heading %>
       <div class="content-heading">
-        <div class="content-inner">
+        <div class="content-inner <%= yield :heading_class %>">
           <%= yield :heading %>
         </div>
       </div>