v2.0.0 - Support for Dash v2.0

This release marks a jump to supporting v2.0 and higher of Dash.

Note that this is not backwards compatible. Please use v1.7.2 or earlier if you need to use a version of Dash less than v2.0

Whilst the intention to leave v1 (both Dash and django-plotly-dash) behind, it is still possible to create releases for v1.x Dash using patches applied to the v1.7.2 tag code, should it prove necessary.