simplify_regex should remove redundant escape sequences outside groups

[Cristi Vîjdea]

django.contrib.admindocs.views.simplify_urls should clean up escapes found outside path parameters. Otherwise, broken URLs with backslashes can be generated and displayed.

This is readily apparent with Django 2's path(), which aggresively escapes everything outside a <parameter> specifier, resulting in a urlpattern with backslash-escaped forward slashes:

>>> simplify_regex(r"^(?P<sport_slug>\w+)/athletes/(?P<athlete_slug>\w+)/$")
'/<sport_slug>/athletes/<athlete_slug>/'
>>> simplify_regex(r"^(?P<sport_slug>\w+)\/athletes\/(?P<athlete_slug>\w+)\/$")
'/<sport_slug>\\/athletes\\/<athlete_slug>\\/'

The second example is what path() would generate in urlpatterns.