Support HTTP Range header

[Syndesi]

The Range header is required if big binary responses, like getting files, should be retrievable in smaller chunks.

See also:

• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Range

Acceptance criteria:

• GET /<id>/file supports Range header.
• Unit tests.
• Feature tests.
• Examples are documented.

[Syndesi]

Blocked by #119 .