Add support for (ticket) email blasts

[aviupadhyayula]

Adds support for email blasts from event owners to ticket holders. I'm envisioning using this for an admin feature as well (ex: OSA wants to send a message to all club leaders, like they did at the start of this semester).

Here's what an event-related email might look like:

[julianweng]

Thanks for taking the lead on this: you might have to add the action name to the relevant code in EventPermission in order to ensure that only club leaders use it but looks good otherwise. Also, would it be possible to take a first crack at the UI? Shouldn't be more than 20 or so loc in the club-specific ticket page.

[aviupadhyayula]

Also, would it be possible to take a first crack at the UI? Shouldn't be more than 20 or so loc in the club-specific ticket page.

Sure, but I think the plan is to have a designer take a crack at the UI before we make any changes or additions to it. We're going to need to change how the ticket dashboard looks to club leaders a lot (add refunds, email blasts, revenue metrics, question answers, etc.). Let me know if that sounds reasonable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.99%. Comparing base (b280dc0) to head (df498d0).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #749      +/-   ##
==========================================
+ Coverage   71.95%   71.99%   +0.04%     
==========================================
  Files          32       32              
  Lines        6953     6964      +11     
==========================================
+ Hits         5003     5014      +11     
  Misses       1950     1950              

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[julianweng]

LGTM other than one nit.