Add option to download (altered) NWB file

[pgleeson]

A potential usecase (before we move to more interactive workspaces on OSBv2) is that a user alters some metadata in the nwbfile object (probably in the console, but potentially through the graphical elements in future, e.g. editable text labels) and wants to save the resultant nwbfile as an actual file.

There could be a Save icon in the top right corner which calls the standard method to generate a new NWB HDF5 file and allow it to be downloaded locally from the browser.

Obvioulsy this would be an issue with large files, but this relatively minor addition would make the application read & write and an easy way for that non-programming PI to make final edits to the NWB files before release...