Add serve_dir

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In order to use web assets such as images in constructing web pages, Tide needs access to the directory containing the assets. This is arranged through Tide's serve_dir function. This function needs the path to the asset directory. See serve in app.rs.

• Arrange for assets to be installed in a known location or locations, with provisions for assets defined by the application, the organization, and tide-disco itself
• Call serve_dir with this location