Playbooks: Ability to add groups to tasks in runs

Groups should appear in the autocomplete under the category of “Groups”. Category order should be “Run Participants”, then “Groups”, then “Not Participating”. Autocomplete can maintain the same UI/UX as it does when groups are shown in the @ mention autocomplete when drafting a post.

When a group is added as a task assignee, add the individual group members as participants to the run (this logic is already in place when individuals are assigned to tasks today)

Adding a user group as an assignee means that new users added to the group automatically get assigned that task, get notifications about it, and are added to the run channel.

For the daily playbook bot notifications about assigned tasks, all members of the group should receive it (the same way the notifications are sent for individuals assigned to tasks today)