refs https://github.com/TryGhost/Team/issues/1294
- added `dashboardHideGettingStarted` feature flag that uses the `user.accessibility` JSON blob
- added `{{toggle-feature}}` helper which returns a function that can be used with the `{{on}}` helper to toggle a feature flag on or off from templates
- hooked up the dismiss behaviour for the dashboard widget using the `toggle-feature` helper
- hid the dashboard when `dashboardHideGettingStarted` feature is enabled
