[Release] Drop jQuery, replace QUnit with Jest, refactor styles to follow BEM and Airbnb styleguide, fixes

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Changes:

• estatico-boilerplate: Removed jQuery
• estatico-boilerplate: Replaced QUnit with Jest, added estatico-jest
• estatico-boilerplate: Refactored styles to use BEM and follow Airbnb styleguide, extended estatico-stylelint config accordingly
• estatico-boilerplate: Moved dev styles and scripts into /preview
• estatico-boilerplate: Optimized global client-side namespace
• estatico-boilerplate: Added --fix to css:lint and js:lint task
• estatico-boilerplate: Optimized slideshow demo
• estatico-eslint: Coupled 'fix' option to --fix flag, disabled by default
• estatico-stylelint: Added airbnb configuration, enabled linting in boilerplate, refactored styles accordingly

Fixes:

• estatico-boilerplate: Fixed skiplink export
• estatico-boilerplate: Fixed task orchestration
• estatico-boilerplate: Added max-width to layout wrapper
• estatico-handlebars: Fixed tests
• estatico-stylelint: Fixed error reporting
• estatico-webpack: Fixed tests