Update the page header and include course image / activity icon

The page header currently (can) contain these elements

• user image (in user contexts)
• site logo (frontpage)
• page title
• navbar (breadcrumb)
• a settings menu (cog)
• page heading button (turn editing on)
• header actions (search on wiki)
• course header added by the course format?

Suggested improvements for the header are:

• add course image on course page
• add activity icon on activity page
• remove border around the header (do not use the .card class)
• add a back to course action on the activity page