Scrum timeline ui

PT story: https://www.pivotaltracker.com/story/show/78315784

- replaced some elements in scrum timeline to match WSO standards (is there a standard???)
- replaced 'Events' link in navigation bar with a dropdown menu with links to `/events` and `/scrums`
- removed "Previous scrums" link from underneath round banners
- refactored some scenarios in `basic_layout.feature`