Improve quality of life for 4-inch phones

Removes extra UI margins < 360px, and allows the tab bar to scroll. Also slightly improves horizontal scrolling behaviour on desktop.

app/assets/javascripts/components/features/ui/components/tabs_bar.jsx

@@ -3,9 +3,8 @@ import { FormattedMessage } from 'react-intl';
 
 const outerStyle = {
   background: '#373b4a',
-  margin: '10px',
   flex: '0 0 auto',
-  marginBottom: '0'
+  overflowY: 'auto'
 };
 
 const tabStyle = {