Fix UI of Slider Field

Current issues:

• The value field is positioned way too high
• The label shows up redundantly above the slider ("Skalierung" to the right of 100 %)
• The vertical margin between slider and slider marks is too small, does not look good
  

Improvements:

• slider width: 600px
• font for the scales (below slider): Roboto, regular, 12px, color #B7B7B7

Dev notes:

• Add-ons specific implementation has solved at least a few issues: https://git.magnolia-cms.com/projects/ADDON/repos/ab-testing/browse/magnolia-abtesting-ui/src/main/java/info/magnolia/abtesting/ui/SliderField.java
• Merge back into one field?