Returns quiz_id in response of create_quiz POST request

[suryabulusu]

Fixes #55

Summary

• Motivation: Returns only the quiz_id instead of the entire created quiz. This reduces load on network.
• Created quiz can be retrieved by sending a GET request at /quiz/{quiz_id} endpoint
• Changes are also made in corresponding app/tests files

[codecov]

Codecov Report

Merging #56 (4de30d0) into main (e26415c) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   95.98%   96.04%   +0.06%     
==========================================
  Files          16       16              
  Lines         473      481       +8     
==========================================
+ Hits          454      462       +8     
  Misses         19       19              

Impacted Files	Coverage Δ
app/models.py	100.00% <100.00%> (ø)
app/routers/quizzes.py	100.00% <100.00%> (ø)
app/tests/base.py	100.00% <100.00%> (ø)
app/tests/test_quizzes.py	100.00% <100.00%> (ø)
app/tests/test_sessions.py	100.00% <100.00%> (ø)

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[pritamps]

Looks good to me, but I will let @deepansh96 approve. I'm only starting to get familiar with quiz ka code