Add floating window for new threads

New threads get a floating window just like new replies have. This only
pops up on the board detail view.

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

board/urls.py

@@ -10,6 +10,7 @@ urlpatterns = [
     path("<slug:url>/", BoardView.as_view(), name="board_detail"),
     path("<slug:url>/page/<int:page>/", BoardView.as_view(), name="board_detail"),
     path("<slug:url>/post/<int:id>/", PostView.as_view(), name="post_detail"),
+    path("<slug:url>/post/create/", PostCreateView.as_view(), name="post_create"),
     path("<slug:url>/reply/<int:id>/", ReplyCreateView.as_view(), name="reply_create"),
     path("post/success/", PostSuccessView.as_view(), name="post_success"),
     # Reports