Auto-install extensions

Squashed version of #9490 

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Automatically install extensions "bundled" with installer on startup. This feature relies on the installer putting **zip files** for "bundled" extensions into the `[install-dir]/auto-install-extensions/` folder.

This is for #9233 

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### Testing Matrix for `auto-install-extensions` folder

#### Should fail silently
* is missing
* is empty
* contains file which has non-zip file extension
* contains file which has non-extension with zip file extension
* contains extension that is already auto-installed
* contains extension that is already manually installed
* contains extension that has a newer version auto-installed
* contains extension that has a newer version manually installed
* contains extension that was already auto-installed, then manually removed

#### Should install extension
* contains extension that is not yet installed (auto or manually)
* contains extension that has an older version auto-installed
* contains extension that has an older version manually installed

Sorry, I missed this line on my last review. This isn't a preference, so it should not be defined on the PreferencesManager. It should either be `PreferencesManager.stateManager.definePreference` or not here at all.