refs https://github.com/TryGhost/Team/issues/1111
refs https://github.com/TryGhost/Team/issues/1103
- moved customize modal from a link on the design screen to the main design screen
- changed modal design to be a full-screen overlay with side bar that emulates standard Admin design
- added toggled groups of settings in sidebar
- added `{{set-has}}` helper for use in conditionals matching when a Set contains an object
- added grouping of theme settings
- dropped unfinished advanced/change theme modals
