Update Django Example Apps

[sdc50]

Update django example apps to work with multiple versions of Django/Channels.

Changes will allow Django example apps to be run with Django/Channels 2 and with Django 3/4 (with Channels 3).

Conditionally imports bokeh_django to enable using Channels 3.

[sdc50]

@philippjfr once we release bokeh_django then we can update the docs to explain how to install it.

[philippjfr]

Worth making these updates even if bokeh_django isn't available yet. Will merge shortly.