Add support for listing all followed sites

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.

The issue does not name files or tests. Start by locating the bot's command handling for followed sites and determine the expected command and output for listing them all. Done means the command lists every followed site so users can identify and remove unwanted notifications.

Written by the indexing model from the issue text.