Handle Slack app_uninstalled callback event

[alysivji]

Closes #324 Closes #330 (was able to knock this out while working on this ticket)

What does this do

Handle app_uninstalled event from Slack workspace

Why are we doing this

Improve user experience

How should this be tested

Wrote tests around this functionality. Also had to modify payloads to match new oauth workflow.

Migrations

• added Cascade delete migrations to all foreign key relationships
• removed user token column as we don't really need it

Dependencies

• Turn on new event_subscription API
  • [x] staging
  • [x] production

Callouts

n/a

[codecov-commenter]

Codecov Report

Merging #331 into master will decrease coverage by 0.08%. The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #331      +/-   ##
==========================================
- Coverage   89.92%   89.83%   -0.09%     
==========================================
  Files          71       71              
  Lines        2491     2499       +8     
==========================================
+ Hits         2240     2245       +5     
- Misses        251      254       +3     

Impacted Files	Coverage Δ
busy_beaver/apps/call_for_proposals/models.py	59.45% <ø> (ø)
busy_beaver/apps/github_integration/models.py	69.38% <ø> (ø)
..._beaver/apps/slack_integration/oauth/oauth_flow.py	100.00% <ø> (ø)
busy_beaver/apps/upcoming_events/models.py	92.85% <ø> (ø)
...eaver/apps/slack_integration/event_subscription.py	93.47% <83.33%> (-2.87%)	:arrow_down:
busy_beaver/apps/slack_integration/models.py	100.00% <100.00%> (ø)

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