Added config item to refine.ini for system property refine.max_form_content_size

When using OpenRefine with super large data sets and using the fingerprint clustering algorithm, I usually get an error (the same error as https://github.com/OpenRefine/OpenRefine/issues/638) when trying to merge and re-cluster a massive data set. I noticed that the max form content size has been added to the system properties in https://github.com/OpenRefine/OpenRefine/pull/876. This PR would make it a lot easier to modify the property by adding an entry into refine.ini, and it is commented out by default.