refs https://github.com/TryGhost/Team/issues/1111
Extracted functionality for listing, downloading, activating, and deleting from the theme controller/template into separate components and services so that they are more composable/reusable in different situations.
- moved theme activation to a new `theme-management` service that uses the `modals` service to open the theme warnings modal or limits upgrade modal as required
- the activate process is a task so that consumers can store a reference to the task instance and cancel it to close any related warning/limit modals (eg, when navigating away from the route or closing the modal that kicked off the process)
- created new-pattern modals for custom theme limit upgrade, theme errors, and delete confirmation so that we can treat them as promises and close where needed from parent
- duplicated theme table component as `<GhThemeTableLabs>` with an actions redesign and a refactor to handle download, activation, and deletion itself making use of the new theme-management service and modals
- fixed some oddities with design modal's transition/modal close handling by simplifying the async behaviour and being more explicit
- added advanced design modal that contains the new theme table component and linked to it from footer of design modal's sidebar
