feat: set initial value for pacing_type in course run create form

[zawan-ila]

PROD-3234

Testing Instructions

1. Create a self paced run for a course
2. Rerun this course by clicking on Add Course Run at the bottom of the page on publisher
3. Verify that the resulting form has self_paced selected by default for the pacing field

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.05%. Comparing base (8c0be58) to head (8499db1).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #997      +/-   ##
==========================================
+ Coverage   67.85%   68.05%   +0.19%     
==========================================
  Files         129      129              
  Lines        3248     3249       +1     
  Branches      943      944       +1     
==========================================
+ Hits         2204     2211       +7     
+ Misses        996      990       -6     
  Partials       48       48              

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