Fix GRIP next uri when pushpin routes use path_beg and replace_beg

[timomeara]

• adds django_eventstream.utils.build_next_uri method to build next uri
• use settings.EVENTSTREAM_PATH_PREPEND value if present to build correct next uri

[jkarneges]

Thanks for the report and PR! The change seems reasonable, although I'd think proxying to any Django app by subpath would have this problem and I wonder if there isn't a more general solution. For example, prepending the value of get_script_prefix. What do you think?

[enzofrnt]

Should we close this @jkarneges ?

[jkarneges]

Let's see if my suggestion would actually work, and if so then we can close this one.