fix: make event refund_policy editable

[snitin315]

For fossasia/open-event-frontend#5029

Refers fossasia/open-event-frontend#4957

[codecov]

Codecov Report

Merging #7253 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development    #7253   +/-   ##
============================================
  Coverage        63.55%   63.55%           
============================================
  Files              259      259           
  Lines            13034    13034           
============================================
  Hits              8284     8284           
  Misses            4750     4750           

Impacted Files	Coverage Δ
app/api/schema/events.py	96.89% <ø> (ø)

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