Fix version 1 exams missing counterInExercise field

[jonboiser]

Summary

1. Adds a conversion function that adds a missing field to some Version 1 Exams
2. Combines conversion functions into one that handles V0 -> v2 and V1 -> V2 conversion.
3. Updates tests with examples from both V0 and V1 (Fixes make sure unit test examples include all exam versions #4940 )

Reviewer guidance

1. Make some exams using the current 0.12 branch, which should create exams that are V1. Edit the Exam models in the shell, or via a REST tool to create Exams whose question_sources items are missing the counterInExercise field and also have a data_model_version field equal 0. Confirm that the problem in 'NaN' in titles of difficult questions #5427 occurs
2. Update to this fork and verify that it is fixed.

References

Fixes #5427

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• Automated test coverage is satisfactory
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[codecov]

Codecov Report

Merging #5431 into release-v0.12.x will increase coverage by 0.13%.
The diff coverage is 71.42%.

Impacted Files	Coverage Δ
...oach/assets/src/modules/questionDetail/handlers.js	0% <ø> (ø)	⬆️
...s/coach/assets/src/modules/questionList/actions.js	0% <0%> (ø)	⬆️
...rc/views/plan/CreateExamPage/CreateExamPreview.vue	0% <0%> (ø)	⬆️
...ns/learn/assets/src/modules/examViewer/handlers.js	0% <0%> (ø)	⬆️
...assets/src/modules/examCreation/selectQuestions.js	85.71% <100%> (+0.52%)	⬆️
kolibri/core/exams/serializers.py	98.93% <100%> (+6.15%)	⬆️
kolibri/core/exams/models.py	97.72% <100%> (ø)	⬆️
kolibri/core/assets/src/exams/utils.js	45.54% <80%> (+22.08%)	⬆️
kolibri/core/assets/src/state/mappers.js	85% <0%> (+45%)	⬆️

[indirectlylit]

my guess/hope is that this will also address #5425

[nucleogenesis]

@jonboiser

I tested the latest commits and they resolved the issues we discussed on Slack. When there are no counter_in_exercise fields then the numbers are displayed as expected.

There appears to be a failing test (which I get locally as well running yarn test): https://travis-ci.org/learningequality/kolibri/jobs/526240872 - once that's resolved then this should be ready to roll.

[nucleogenesis]

@jonboiser Looks great!