Add option to download (altered) NWB file

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...

The issue names the nwbfile object and a possible Save icon in the top-right corner, but no files, tests, or entry points. Start by locating how the browser currently handles the nwbfile and any existing download or serialization paths. Done means altered metadata can be written as a new NWB HDF5 file and downloaded locally.

Written by the indexing model from the issue text.