Limit certain interactions to Slack users with the Admin role

[jcpsantiago]

The app shouldn't allow all users from performing certain actions e.g. creating archives of private channels.

Slack users have roles, and we should make use of them: https://slack.com/help/articles/360018112273-Types-of-roles-in-Slack

The user type has some useful fields (https://api.slack.com/types/user):

• is_admin
• is_owner
• is_primary_owner

these let us know what is user's role within the slack workspace, and we can use them to determine if a user is allowed certain actions.

We could also restrict users based on whether they originally created the channel, or not.
This would add more flexibility in the "archiving private channel" scenario, by allowing the creator to setup archives, in addition to the workspace admins/owners.