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

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`