refs https://github.com/TryGhost/Team/issues/1107
- allowing empty colors introduced some edge cases around theme usage and default values
- it makes more sense to force a color setting to always have a color, if a theme wants to make usage of that color optional then it should use a separate boolean or select setting to control that
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'`
no issue
- added `{{humanize-setting-key}}` helper that uses the base `humanize` output but then forces capitalization for known acronyms
- updated `<CustomThemeSettings::Select>` to use the new helper
refs https://github.com/TryGhost/Team/issues/1097
- added `customThemeSettings.keyValueObject` which returns a plain object with setting values assign to the setting keys on the object
- updated customize modal's preview data generation to use `customThemeSettings.keyValueObject` to assign a JSON-stringified version to the `custom` param on the `x-ghost-preview` header
- updated theme settings `<Select>` component to call the passed through `updatePreview()` action when the value changes
refs https://github.com/TryGhost/Team/issues/1070
- split select form component into it's own component so it's cleaner when we get to additional setting types
- added change handler that updates the setting record's value when a new option is selected
- added `.isDirty` to the custom-theme-settings service so we can warn of unsaved changes and revert any changed values when needed
- added save of custom theme settings to the customize design modal's save routine
- added missing `notifications` service import to customize design controller
