Swap transitions with finite-state-machine

[alysivji]

Closes #258. This is the last part of the Settings MVP

What does this do

• use finite-state-machine instead of transitions as it provides a simpler API
• modify the onboarding state machine to work with new web interface for settings

Why are we doing this

Since we moved settings to a web interface, we need to redo the state machine

How should this be tested

Let's write an end to end test with multiple steps

Migrations

n/a

Dependencies

• [x] update Slack staging and production installation callback urls
• [x] update GitHub staging and production callback urls

Callouts

• [x] fix slack installation callback url

[codecov-commenter]

Codecov Report

Merging #282 into master will increase coverage by 0.78%. The diff coverage is 91.76%.

@@            Coverage Diff             @@
##           master     #282      +/-   ##
==========================================
+ Coverage   96.03%   96.81%   +0.78%     
==========================================
  Files          70       70              
  Lines        1941     1947       +6     
==========================================
+ Hits         1864     1885      +21     
+ Misses         77       62      -15     

Impacted Files	Coverage Δ
...usy_beaver/apps/github_integration/api/__init__.py	100.00% <ø> (ø)
busy_beaver/apps/github_integration/models.py	100.00% <ø> (ø)
busy_beaver/apps/slack_integration/api/__init__.py	100.00% <ø> (ø)
busy_beaver/common/wrappers/slack.py	94.82% <ø> (-0.26%)	:arrow_down:
busy_beaver/apps/web/views.py	83.33% <50.00%> (+26.19%)	:arrow_up:
...eaver/apps/slack_integration/event_subscription.py	92.70% <83.33%> (-2.25%)	:arrow_down:
busy_beaver/app.py	94.00% <100.00%> (+0.12%)	:arrow_up:
busy_beaver/apps/slack_integration/api/oauth.py	100.00% <100.00%> (ø)
..._beaver/apps/slack_integration/oauth/oauth_flow.py	100.00% <100.00%> (ø)
...aver/apps/slack_integration/oauth/state_machine.py	100.00% <100.00%> (+13.51%)	:arrow_up:
... and 2 more

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