Re-use official static3 package as 0.6.1 fixed the bug we found

Use own dj-static fork as long as our patch is in review
Remove setting for FORCE_SCRIPT_NAME

doc/admin/config.rst

@@ -25,7 +25,6 @@ Example::
     instance_name=pretix.de
     global_registration=off
     site_url=http://localhost
-    scriptname=/presale/
     currency=EUR
     cookiedomain=.pretix.de
     securecookie=on
@@ -37,9 +36,6 @@ Example::
     Whether or not this installation supports global user accounts (in addition to
     event-bound accounts). Defaults to ``True``.
 
-``scriptname``
-    The path pretix runs at, if it does not run under its own subdomain.
-
 ``site_url``
     The installation's full URL, without a trailing slash.