Allow editing of nbgrader config through the formgrader

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It appears that if I change something in nbgrader_config.py, the formgrader is not aware of the changes until I stop and restart my server.

For example, validating an assignment told me that I needed to change c.ClearSolutions.code_stub. I did. But the formgrader did not know until I restarted the service.

A better design might be:

1. allow editing config settings from the formgrader itself
2. at least allow a "refresh" (say through a button) that would re-read the nbgrader_config.py file

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Re-reading the nbgrader_config.py files (and rebuilding gui) would solve a few issues. But it would need to:

1. read the root nbgrader_config.py file to see which c.CourseDirectory.root to use
2. re-read c.CourseDirectory.root/nbgrader_config.py file

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Related to #798