Web frontend for `tracker endpoint`, for query developer use

Related to #420 (closed) but focusing on a web frontend rather than CLI.

Extend the HTTP server exposed by tracker endpoint --http-port to support the following:

• Editing multiline SPARQL queries
• Executing them (via the HTTP SPARQL endpoint)
• Setting parameter values for queries with ?parameters
• Error highlighting
• Warning highlighting (requires a SPARQL linter, to be developed separately)
• Displaying query results in tabular form
• Displaying all info for a given resource, and navigating to different resources
• A series of example queries to copy, paste and adapt
• Links to documentation

This is a planning ticket which requires scoping and creating further ticks.

Design references:

• GraphQL Playground
• Datasette