Extension permissions after uninstalling

[arrenv]

Steps to reproduce

1. Install an extension.
2. Check the Permissions page.
3. Note that the extension appears in the Permissions list, with the required permissions.
4. Uninstall the extension.
5. Check the Permissions page.
6. Note that the extension continues to remain in the list with the same permissions.

Expected behaviour

• Uninstalled extensions should not have permissions or remain on the Permissions page.

Actual behaviour

• Despite uninstalling an extension, the extension still shows on the Permissions list with permissions.

[rdig]

This is a larger issues that affects all extensions, and this will most likely mean (until we switch to multicalls) that we'll have to send an extra transaction to remove the extension's permissions