Tabs: update tab interface medium/large screen appearance

1. Make .active tab more obvious by removing its bottom
   border and setting its background colour to white.
2. Increase padding/margin of tabs.
3. Remove tab panel bottom shadow and corner rounding.
4. Match tab panel border colour with tab border colour.
Merge pull request #4486 from patheard/tabs

Tabs: update tab interface medium/large screen appearance