Review spec/javascripts folder and unify the test suite with Jest only

We are using 3 unit testing libraries: Karma, Jasmine and Jest.

Jest is set up for *.jsx files which should contain React Javascript but there are some files not containing any React that have the extension .jsx only to be processed by Jest and not Karma.

We should simplify our test suite and use only Jest both for JS and JSX. Tasks:

• Review that .jsx files only contain React Javascript, otherwise change it to .js
• Remove all karma and jasmine config/setup files
• Uninstall karma, jasmine and all associated modules
• Fix any failing tests due to changing to Jest
• Add // @flow wherever it's missing
• Review actual test cases and update those that are too simple or outdated