add a new *Metadata* tab

[gregcaporaso]

To make sure that it's clear to users that their metadata is in their Visualizations, we should integrate a new Metadata tab. The most basic form of this might just provide links that users can use to download each of the different metadata artifacts in their provenance. Even better would be if there was a summary that listed all of the metadata columns that are included, like we do in qiime tools inspect-metadata.