Ruusukivi map slider

[ruusukivi]

Closes #71

• Fixed the timeline to be more responsive - should now work both in mobile and desktop even when the user changes the font size between 80% - 125%
• Refactored code: MUI-theme in assets/theme.js, separate component for Accessibility dropdown
• Added basic Jest tests for Header and Accessibility components

I could not find where to set the "Current" label in timeslider so that the backend won´t break -> made a bit quick-and-dirty fix in the component

[JoJoensuu]

Works. Tested different window widths and zoom levels. Past 125% window zoom text below the slides goes partially below the map. Might want to address in later PRs.