* Added separate post types for system_join_leave and system_add_remove posts
* Added username prop to channel deleted posts
* Changed slack import to properly generate the new join/leave messages
* Added username prop to channel update posts
* Moved Post(AddTo/RemoveFrom)ChanneMessage into app package
This commit provides the first part of the bulk import system.
The CLI command is provided, complete with validation & apply modes. All
the basic properties of Teams and Channels can be imported. Users &
Posts will follow separately in a future commit.
This fixes the issue where the channel fails to Import from Slack if
there is already a channel with the same name on Mattermost that has
been deleted.
