Restructure on-boarding from Slack messages to website

[alysivji]

Part of #296
Also some work for #290

What does this do

• restructure onboarding flow from Slack-based workflow to Slack message pointing installers to settings website
• remove onboarding state machine since we don't need it anymore
• started refactoring settings pages to get ready for settings page for Upcoming Events

Why are we doing this

To make it easier for admins to set up Busy Beaver.

How should this be tested

Removed tests, added some tests.

Migrations

We removed the state field

Dependencies

n/a

Callouts

Still need to improve actual text sent to users and shown to users during first-time flow.

[alysivji]

Originally started working on the Upcoming Events configuration, but it makes sense to simplify onboarding before doing that work

[codecov-commenter]

Codecov Report

Merging #301 into master will decrease coverage by 2.20%.
The diff coverage is 41.37%.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
- Coverage   94.72%   92.51%   -2.21%     
==========================================
  Files          70       69       -1     
  Lines        2065     2031      -34     
==========================================
- Hits         1956     1879      -77     
- Misses        109      152      +43     

Impacted Files	Coverage Δ
busy_beaver/apps/slack_integration/models.py	100.00% <ø> (ø)
...usy_beaver/apps/upcoming_events/upcoming_events.py	100.00% <ø> (ø)
busy_beaver/apps/web/views.py	40.69% <15.62%> (-29.90%)	⬇️
...sy_beaver/apps/slack_integration/oauth/workflow.py	70.49% <22.22%> (-22.27%)	⬇️
busy_beaver/apps/slack_integration/api/oauth.py	100.00% <100.00%> (ø)
...eaver/apps/slack_integration/event_subscription.py	96.25% <100.00%> (+0.55%)	⬆️
busy_beaver/apps/web/forms.py	100.00% <100.00%> (ø)
busy_beaver/common/wrappers/slack.py	95.16% <100.00%> (+0.33%)	⬆️

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