Add unit tests with `jest`

[emibcn]

Add unit tests for components:

• src/
  • App
  • About
  • AppThemeProvider
  • asyncComponent
  • Dashboard
  • ErrorCatcher
  • index
  • Language
  • Loading
  • MenuItems
  • Menu
  • ModalRouter
  • ModalRouterWithRoutes
  • Slider
  • Throtle
  • withDocumentVisibility
• src/i18n/
• src/Backend/
  • IndexesHandler
  • Provider
  • src/Backend/Base/
    • Cache
    • Common
    • ContextCreator
    • GHPages
    • withHandlerGenerator
  • src/Backend/Bcn/
  • src/Backend/Charts/
  • src/Backend/Maps/
• src/Widget/

Current coverage

[emibcn]

If anyone wants to help, don't hesitate to comment it here to coordinate: we can work together by testing different components.