* For #1063 - Adds feature flag and pref for new tab tray

* For #1063 - Swaps add tab to tab tray button when newTabTray is enabled

* For #1063 - Creates hidden preference to use new tab tray

* For #1063 - Hides tabs on home screen when setting is enabled

* For #1063 - Navigate to new tab tray from browser with setting enabled

* For #1063 - Fixes regression where we dont show the new tab message with no tabs and no collections

* For #1063 - Fixes crash when toggling to private mode on the home screen

* For #1063 - combines both settings. Cleans up lint errors