Making metadata fields optional

[pritamps]

Fixes #49

Summary

All metadata fields should be optional since these will be org-specific or domain-specific

Tested by running locally and creating quiz

[codecov]

Codecov Report

Merging #50 (45063f0) into main (30e7854) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #50   +/-   ##
=======================================
  Coverage   98.13%   98.13%           
=======================================
  Files          14       14           
  Lines         429      429           
=======================================
  Hits          421      421           
  Misses          8        8           

Impacted Files	Coverage Δ
app/models.py	100.00% <100.00%> (ø)

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