* Prevent autoloading an existing draft/future changeset when theme not active.
* Add missing notifications container to Themes panel.
* Remove deactivation of themes panel when selected status is not publish.
* Show notification in Themes panel when themes cannot be previewed and disable preview buttons.
* Reject installTheme call when theme preview not available.
* Return promise from installTheme and eliminate use of global events in favor of promises.
Props westonruter, melchoyce, zoonini.
See #37661, #39896.
Fixes#42126.
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