Release v2.1.0

Major update to use third-party library rangeslider.js (#887).

Note that this updated version works best with v2.0.2 of the vanilla theme - but will still function with older versions.

Also includes the following fixes/changes:

• fix for error being thrown when _correctRange not defined in json (#888)
• fix for some RTL layout issues (#913)
• update to allow for the 'step' of the slider scale to be changed (#862)
• update properties.schema:
  • with translatable properties in preparation for localisation support (#1066)
  • to allow for some of the aria labels to be inherited from the 'global' aria labels when using the authoring tool (#1111)
• fix for issue of being unable to click the scale numbers in completed slider components in subsequent sessions (#1060)