Add support for checkbox questions

[dalmia]

Fixes #260
Fixes #261

Summary

• added checkbox as another valid question type
• changed data type of answer in session answer to JSONField
• changed data type of correct_answer in question to JSONField
• updates the README accompanying a plio's report to make it clearer
• in the report, we were giving correct_answer 1-indexed but session answer as 0-indexed. Made it 1-indexed for both now.

Test Plan

• Wrote tests
• Tested locally
• Tested on staging
• Tested on production
• If changes in DB, update DB schema and BigQuery (staging and prod)

[codecov]

Codecov Report

Merging #258 (55734b9) into master (6f36eb6) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #258      +/-   ##
==========================================
+ Coverage   96.96%   96.99%   +0.03%     
==========================================
  Files         144      147       +3     
  Lines        2667     2696      +29     
==========================================
+ Hits         2586     2615      +29     
  Misses         81       81              

Impacted Files	Coverage Δ
plio/config.py	100.00% <ø> (ø)
plio/queries.py	100.00% <ø> (ø)
entries/migrations/0028_auto_20211130_0828.py	100.00% <100.00%> (ø)
entries/models.py	98.07% <100.00%> (ø)
plio/migrations/0027_auto_20210902_1049.py	100.00% <100.00%> (ø)
plio/migrations/0028_auto_20210902_1120.py	100.00% <100.00%> (ø)
plio/models.py	94.68% <100.00%> (ø)
plio/views.py	95.38% <100.00%> (+0.44%)	⬆️

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