Sharing URLs

We need to be able to share individual dashboards with hard-to-guess URLs (UUIDs or something, probably) and possibly different authentication in front of those.

I'd like to read the same configs for these shared dashboards as for the general ones. Perhaps we build a sharing URL for each dashboard that has a `share_id` config parameter or something and include a link to the shared version from the index and/or the non-shared dashboard page?