Added difficulty (Associated Issue: #537)

[martinh-dev]

Associated Issue: #537

Summary of Changes

• added difficulty to every rebus (from 1 to 5)
• translate difficulty number into difficulty level (1=easy, 2=basic, 3=intermediate, 4=hard, 5=very hard)
• print difficulty level on the top right corner of the rebus card
• added new span to the jest test

[coveralls]

Pull Request Test Coverage Report for Build 890

• 2 of 7 (28.57%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.8%) to 84.3%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/js/components/Difficulty.js	2	7	28.57%

Totals
Change from base Build 514:	-2.8%
Covered Lines:	160
Relevant Lines:	187

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💛 - Coveralls

[martinh-dev]

Coverage check is always not successful. What should i change? Is it nevertheless mergeable ?