[FEATURE] support regexes as URL paths in Django

[PeteFein]

Is your feature request related to a problem? Please describe.
Django server integration only supports fixed-string URL paths in myproject.urls.bokeh_apps. This makes it impossible to customize those apps based on URL arguments - I'd like to construct app instances using database objects referenced in URL slugs.

Describe the solution you'd like
Support regexes in bokeh_apps as URL patterns.

Describe alternatives you've considered
Bypassing bokeh_apps entirely and constructing channel's routing by hand everywhere I need it. This would be sad.

Similar behaviour was requested in bokeh/bokeh-django#4.

Additional context
PR forthcoming. Also tweaks bokeh.server.contexts._RequestProxy to better support Django-style request dictionaries.

[bryevdv]

@PeteFein Is there anything else in bokeh/bokeh-django#4? If not we can close it. If so, can you make individual issues for anything left (then we can close it)?

[PeteFein]

@PeteFein Is there anything else in bokeh/bokeh-django#4? If not we can close it. If so, can you make individual issues for anything left (then we can close it)?

bokeh/bokeh-django#4 isn't my issue, and I'm not fully sure what's being requested there (it's quite long), so that's probably best left to someone else