When things go wrong with extensions, it's difficult to solve without

going into the database. This change aims to help with that by offering

management commands for enabling and disabling extensions, and for

listing all available (or all enabled) extensions.
'enable-extension' and 'disable-extension' take an extension ID, and

will enable or disable.
'list-extensions' will by default list all registered extensions.

Passing --enabled will limit this to enabled extensions.

Disabled an extension, checked that it was disabled, and re-enabled it.
Tested list-extensions with and without --enabled.