Restore the filebrowser icon for file fields.

Files taken from https://github.com/wardi/django-filebrowser-no-grappelli,
which does work properly but isn't in sync anymore with the sehmaschine development line.

This undoes the changes in sehmaschine/django-filebrowser@abad871f9584f2599552612368328d4ceb363f5a
Fix filebrowser button for newly added inlines

This __prefix__ is only replaced in "id" fields by admin/js/inlines.min.js.
Instead, read the ID dynamically so it's always correct.
Add fake model to show filebrowser in admin dashboard
Settings: escape extension dots in default EXCLUDE list, avoid empty entry.
Set xframe_options_sameorigin policy on filebrowser views.
Merge pull request #5 from vdboor/master

General improvements for the filebrowser in the Django admin