When computing the file size of a submission, ignored files are included

[tmetzl]

One can set the maximum directory size of a submission via CourseDirectory.max_dir_size. It is also possible to ignore files and directories via CourseDirectory.ignore.
In the do_copy method of the exchange the size is computed including ignored files and directories here.
This causes nbgrader to reject submissions that would be okay in size if there are ignored files that push the directory size over the limit.

Steps to reproduce:

1. Fetch an assignment
2. Create a large file > max_dir_size and put it in one of the directories ignored by the course directory (feedback, .ipynb_checkpoints, etc).
3. Try to submit