Make codebase compatible with Dash 2.0 release

As well as #351, there is also a need to make django-plotly-dash work with the recent v2.x release(s) of Dash. One issue has already been reported as #353

[IDepla]

I think the problem is with the dash package, It is bypassing the kwargs so you lose the context that you injected

yourenvironment/dash/_callback.py

//line 142
def add_context(*args, **kwargs):
            output_spec = kwargs.pop("outputs_list")
            _validate.validate_output_spec(insert_output, output_spec, Output)

            func_args, func_kwargs = _validate.validate_and_group_input_args(
                args, inputs_state_indices
            )

            # don't touch the comment on the next line - used by debugger
            output_value = func(*func_args, **func_kwargs)  # %% callback invoked %%

I tried (python3.6) adding

def add_context(*args, **kwargs):
            output_spec = kwargs.pop("outputs_list")
            _validate.validate_output_spec(insert_output, output_spec, Output)

            func_args, func_kwargs = _validate.validate_and_group_input_args(
                args, inputs_state_indices
            )
           mergedic={**func_kwargs, **kwargs}
            # don't touch the comment on the next line - used by debugger
            output_value = func(*func_args, **mergedic)  # %% callback invoked %%

and helps my problem (no user in kwargs).

[GibbsConsulting]

v2.0.0 is now out!