Add support for shared channels

Since shared channels only work with the new conversations API, a new
class of SlackChannel and API type were added to support them. Also
`rtm.start` doesn't include external users so their information needs to
be fetched individually when getting the channel history.

To distinguish external users from team members their nicks get appended
with an asterisk, which can be changed with a new setting. They also
appear in a new "External" group in the nicklist since their presences
can't be fetched with `users.getPresence`.

Unfortunately `conversations.history` doesn't include thread messages so
threads won't show up in shared channels.

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