Consider allowing config parameters to be set after initialization

For example the parameters 'url_base_pathname', 'routes_pathname_prefix', 'requests_pathname_prefix' currently need to be set in the creation of the dash object (or in envvars). To me they don't sound like they should be associated with the function that *creates* the Dash object, because it couples the dash object right from the beginning to how it's going to be hosted. That feels like something that should either be added into the Dash object after creation, or provided to the app$run_server() function. Perhaps some sort of a "config" system, where config parameters can be added either in the constructor or in run_serve() or any time in between using app$modify_config().

Just some thoughts for later.

No files or tests are named. Start by tracing where the Dash object is created and where app$run_server() consumes url_base_pathname, routes_pathname_prefix, and requests_pathname_prefix, then clarify whether post-initialization, environment-variable, or run-time configuration is required before defining what done means.

Written by the indexing model from the issue text.