Add special image for URL sharing

When sharing a URL with a state already in it, the sharing preview image should reflect that.

This will be difficult because our HTML is static, and interpreters won't be running our JS. We'd have to serve the HTML dynamically. This fact makes this issue low-priority.

Start by reviewing the static HTML used for URL sharing and how state is represented in the shared URL. Done means the sharing preview image reflects the existing URL state, which the issue notes may require serving HTML dynamically because interpreters will not run the JavaScript.

Written by the indexing model from the issue text.