Ticket drop time UI support, unapproved club event visibility changes

[julianweng]

• Add UI support for ticket drop times
• Add ticket_drop_time validation in event serializer based on event end time
• Prevent retroactive editing of ticket drop time after tickets have been sold (plus handling of edge case where ticket has been added to cart but not bought before ticket drop time is pushed back)
• Prevent unapproved clubs from selling tickets (add_to_cart)
• Prevent clubs which have been archived from selling existing tickets
• Allow leaders of unapproved clubs to create and view events (with the warning that such events are not visible to the public until the club is approved), allowing for "draft" functionality (also currently, the event just disappears after submission on the frontend for unapproved clubs...)
• Standardize spelling of "publicly" in the codebase (alternative being "publically")
• Add button to view public event page within event editing page
• Show all clubs in /directory which have been approved at some point

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 72.01%. Comparing base (b280dc0) to head (8d0adca).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
backend/clubs/views.py	80.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #750      +/-   ##
==========================================
+ Coverage   71.95%   72.01%   +0.05%     
==========================================
  Files          32       32              
  Lines        6953     6967      +14     
==========================================
+ Hits         5003     5017      +14     
  Misses       1950     1950              

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