refs #7983 let plugins add or remove fields to websites and better settings api

CHANGELOG.md

core/API/DataTableManipulator/ReportTotalsCalculator.php

core/API/DataTablePostProcessor.php

core/DataTable/Filter/PivotByDimension.php

core/DataTable/Renderer/Xml.php

core/Db/Schema/Mysql.php

core/Http/ControllerResolver.php

core/Measurable/Measurable.php

core/Measurable/MeasurableSetting.php

core/Measurable/MeasurableSettings.php

core/Measurable/Settings/Storage.php

core/Measurable/Type.php

core/Measurable/Type/TypeManager.php

core/Menu/MenuAbstract.php

core/Notification/Manager.php

core/Plugin.php

core/Plugin/API.php

core/Plugin/Controller.php

core/Plugin/Manager.php

core/Plugin/Menu.php