Add support for group chats

[boxdot]

Behavior should be the following:

• When the bot is added to a group chat, it accepts messages from any user, when a user explicitly uses bot's name. Commands are then interpreted in the same way, as in chats with a single user.
• Notifications are sent to the group chat when the bot is enabled, and a notification from Gerrit for any user in the group chat arrives.

What needs to be implemented:

• Spark messages should be aware of their origin: single-user chat or group chat.
• Actions should work even if the name of the bot is present.
• When a new user enters/leaves the group chat, the bot needs to be aware of this.
• The User type of the bot needs to distinguish between Users and Groups.

This feature will most likely change the storage format of the state.

[gferon]

One more use-case for this: when you add the bot to a group chat, add the ability to give a custom filter for this chat.

e.g. if you are a member of a build scripts group chat, you wanna know when a commit is merged in a repo, but you don't care about CI feedback on said repo.