Update PostModifyForm to use permissions to create its fields

Fields are only displayed via the PostModifyForm if the user has
specific permissions to do things, like set stickies.

Also, add PostModifySuccessView that will close the modify window when
the process is complete.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

board/urls.py

@@ -22,6 +22,9 @@ urlpatterns = [
     path("banned", BannedView.as_view(), name="banned"),
     # Other moderation pages
     path("modify/<int:pk>/", PostModifyView.as_view(), name="post_modify"),
+    path(
+        "modify/success/", PostModifySuccessView.as_view(), name="post_modify_success"
+    ),
 ]
 # TODO - make this conditional so we can serve images up with whatever server we want
 urlpatterns += static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)