in exporting, handle binary files from server routes

[Conduitry]

Resolves #1103, resolves #1104. Builds on work in and supersedes #938 and #1105.

This is basically #1105, with conflicts with master resolved, code style adjusted, and with another case in get_server_route_handler of dangerous buffer-to-string conversion removed.

This lets you follow <img src>, <source src>, and <source srcset> attributes when exporting, and to also correctly handle (without encoding corruption) when these are server routes that return binary files.

[benmccann]

this looks great. very thorough job testing!

[HighestTide]

This currently results in kinda-expected, but kinda-annoying results if you have endpoints that return images without the route having the correct extension. Would be it be worth considering appending the "correct" extension for image files?

It is pretty simple to work around this by just adding the extension manually on the markup side and removing it in the route handler, but it seems like a simple fix could cover a lot of use cases and prevent confusion with exports not matching the running application. I'm not sure what the Sapper philosophy is when it comes to manipulating stuff like that, though.

Another alternative fix could just be to print a warning during export.

Example url based on the actual example I had:
/remote_asset/https%3A%2F%2Fexample.com%2Fasset.svg%3Fcompress%3D50
(https://example.com/asset.svg?compress=50)