update .env.example template for clarity (Resolves #429)

[jiachen247]

• updated .env.example to contain sensible defaults
• parsing of boolean value for REACT_APP_USE_BACKEND

[coveralls]

Pull Request Test Coverage Report for Build 1075

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• 643 unchanged lines in 60 files lost coverage.
• Overall coverage decreased (-18.2%) to 7.878%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/utils/constants.ts	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/components/commons/index.tsx	1	0.0%
src/utils/castBackend.ts	1	0.0%
src/components/workspace/side-content/ListVisualizer.tsx	2	0.0%
src/utils/paramParseHelpers.ts	2	42.86%
src/components/academy/grading/GradingHistory.tsx	3	0.0%
src/components/workspace/side-content/ToneMatrix.tsx	3	0.0%
src/utils/notification.ts	4	0.0%
src/containers/ProfileContainer.ts	4	0.0%
src/reducers/externalLibraries.ts	4	0.0%
src/components/workspace/side-content/index.tsx	4	0.0%

Totals
Change from base Build 1073:	-18.2%
Covered Lines:	279
Relevant Lines:	2982

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💛 - Coveralls

[podocarp]

Works!

[martin-henz]

Very low test coverage that will require a concerted effort. This particular decrease in test coverage seems spurious and should not hold back this PR.