refs https://github.com/TryGhost/Team/issues/1107
- uses the same type of color picker used elsewhere in Admin with some minor changes to allow blank values
- when value is blank it adjusts the background color block to have a border and a diagonal red line to represent no color being present and makes the native color picker that sits on top transparent because native color pickers _always_ have a color
- has it's own invalid handling/display because we're not dealing with a model that has our typical validation at the moment
refs https://github.com/TryGhost/Team/issues/1106
- removed `'string'` from the `attr()` definition for `customThemeSetting.value` so that we're not coercing the value from the API into something that doesn't work when applied to the `checked` attribute on checkbox inputs
- added initial `<CustomThemeSettings::Boolean>` component and used it for display in theme setting forms when `type === 'boolean'`
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
