Add management commands for enabling, disabling, and listing extensions.

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.