refs: https://github.com/TryGhost/Team/issues/1373
- Simplify the member details link on single member activity
- Adjust all the activity scrolling to work better at all sizes
- Make the header work properly with truncating at narrower viewports
- Remove unnecessary CSS and classes
- Remove the top line from the member detail box
- Move the member detail box so it stays when scrolling
- Make the empty state work with new member detail box position
- Better scrolling full width whilst not having full width like members
- Fix the subscriptions event icon which was a white box in dark mode
- Adjustments to global gh-list-scrolling styles at narrower viewports
- Test fully on narrower viewports and major browsers
