Upload data files during node config

[reddigari]

(This includes the commits from #40, so it will look much cleaner after that one is merged.)

When file is selected from configuration menu, it is uploaded to the server and saved with the filename: <node ID>-<original name>.<original_extension>. The configuration form submit button is disabled until the server responds. The parameter to which the file corresponds (e.g. filepath_or_buffer is given the value of the filename on the server.

This brought to my attention a couple of execution bugs:

• ~The filename saved in the node options doesn't have the /tmp prefix, so pandas cannot find it. We will need to add a step in the node update view that replaces the value with the fully qualified FileStorage name~.
• ~The description field is inserted in the node options by the browser, and since the pandas function is called with (**options), it complains that description is not a valid keyword arg.~

These issues are causing me to cool on the "pass the options dict as kwargs" strategy overall. Thoughts?

[reddigari]

• Altered create_node view helper to replace options representing files with the fully-specified FileStorage path so execute() can find the file
• Fixed ReadCsvNode.execute() to not pass the description as a keyword arg.

Execution works!! (at least on ReadCsvNode)