Allow JSON encoder to be set

[GibbsConsulting]

The JSON encoder used to send responses (plotly.utils.PlotlyJSONEncoder) does not currently permit the use of lazy string evaluation in Django, such as for translation of messages, as described in #404

The code in dash_wrapper.py needs to use something derived from this encoder that also can leverage the Django lazy text methods.

[delsim]

See PR #408

[GibbsConsulting]

Now addressed in v2.1.0