Add support for listing all followed sites

[rachelmcr]

It would be great to support a command that lists all the sites you're following, for example to clean up a channel/group that's getting a lot of extraneous notifications.

[markryall]

Yes - I was thinking the same thing.

This is especially important as when you follow some sites, the subscription will be created for a different domain (eg. following a non primary domain).

The XMPP service already supports this so it should be reasonably straightforward