ref(issue-details): Fix some styling with streamlined header

[leeandher]

This PR makes a few changes to the header portion of the event nav:

• overflow long values for message
• align stats with title/message as a grid
• remove divider after level
• makes action buttons bigger again (sorry richard)
• consistent border color on event panel
• simpler dom for header (removed EventMessage and EventGroupOrTitle), less elements for flexboxing actions
• Moves sidebar button and participant list