feat: add submit btn click event for default register page

[zainab-amir]

Description

To compare the control and variation group for the A/B test where we are measuring the impact of removing username from registration form, added event to default "Register for free" button clicks.

JIRA

https://2u-internal.atlassian.net/browse/VAN-1827

How Has This Been Tested?

Locally

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.33%. Comparing base (e1f4898) to head (4140608).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1176      +/-   ##
==========================================
+ Coverage   84.23%   84.33%   +0.09%     
==========================================
  Files         127      127              
  Lines        2392     2394       +2     
  Branches      678      680       +2     
==========================================
+ Hits         2015     2019       +4     
+ Misses        357      355       -2     
  Partials       20       20              

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