Improve Upcoming Events Feature workflow

[alysivji]

Closes #313 Closes #289

What does this do

Cleaning up the flow around how the Upcoming Events feature works.

• when a group is added, upcoming events for that group get added to the database
• can use the Upcoming Events feature without having a periodic posting CRON job set up

Why are we doing this

Improving user experience

How should this be tested

Added tests; also tested manually

Migrations

• made a column nullable

Dependencies

n/a

Callouts

n/a

[codecov-commenter]

Codecov Report

Merging #316 into master will increase coverage by 0.09%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
+ Coverage   92.25%   92.35%   +0.09%     
==========================================
  Files          70       70              
  Lines        2286     2315      +29     
==========================================
+ Hits         2109     2138      +29     
  Misses        177      177              

Impacted Files	Coverage Δ
busy_beaver/apps/upcoming_events/models.py	92.85% <100.00%> (+0.17%)	:arrow_up:
busy_beaver/apps/upcoming_events/sync_database.py	100.00% <100.00%> (ø)
busy_beaver/apps/upcoming_events/workflow.py	62.50% <100.00%> (+19.25%)	:arrow_up:
busy_beaver/apps/web/views.py	73.76% <100.00%> (+0.94%)	:arrow_up:

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