Allows query params in precomputed chunks

[Tomaz-Vieira]

• Creates the ParsedUrl class to streamline handling URLs
• uses ParsedUrl everywhere the PrecomputedChunks datasource composes URLs, concatenating paths and preserving query strings

Fixes #316

Needs #320

TODO:

• Wait for legal to give me the green light on licensing/CLA and for http_path_completion: Loosens check for content-type #320 to be merged
• Get some architectural feedback and comments =)
• Some more testing

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[jbms]

Can we use the existing URL class already provided by browsers, or is it unsuitable for some reason?

https://developer.mozilla.org/en-US/docs/Web/API/URL

[Tomaz-Vieira]

I figured eventually ParsedUrl (or whatever other name you might want to give it) could be used to handle URLs in more places throughout the code where URL manipulation is required. The native implementation would be insufficient because it silently breaks on funny protocols:

new URL("gs://example.com/some/path")
// produces URL { (...) host: "", hostname: "", port: "", pathname: "//example.com/some/path"}
// note the empty hostname, which was interpreted as being part of the pathname

So even if we extended URL, we'd still need to do the parsing ourselves.

Still, if we split handling the special Neuroglancer URLs (e.g. precomputed://http://..., gs://) from the expanded, canonical URLs (i.e.: https?:), then extending the native URL would be enough. But I think unifying all URL handling under a single class would be simpler to follow.

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