Slack group sync

[sywilliamson]

What

• Adds members to the active and members Slack usergroups when they hit the "Become Active" button
• Adds members to the active and members Slack usergroups when they're added as an on-floor member
• Removes all members from the active, frosh, and members Slack usergroups except for the Evals director
• Added slack.py which allows interfacing with CSH Slack via the slack_sdk
• Modified ldap.py to provide a dedicated function to fetch Slack UIDs from CSH Members

Why

See issue #606

Test Plan

Local deployment. Verified slack.py functions worked with a private Slack workspace. Tested functionality by pressing the become active button and new year button, and by creating a new on-floor member.

Env Vars

• Added "SLACK_APP_TOKEN" to create a WebClient so usergroups can be managed by slack.py

Documentation

I'm not aware of any documentation that I have made out of date.

Checklist

• Tested all changes locally