Date/time JSON and files management

[pihart]

• Save export date/time in json (give it a new version number)
  • Resolves Save datetime in JSON #93
• Refactor pages.ts by moving operations to files.ts
• Refactor jsonwriter
• Refactor timestring method

[pihart]

Maybe this should actually return an object of a new type FileUIDownloader with the data

{
  name: `qboard-${FileUI.timeStringNow()}.json`,
  fileURL,
  revokeURL,
}

Then FileUI::download should accept an object of this type, and any function calling JSONWriter::download should take the result and call its own FileUI::download. I'd probably rename JSONWriter::download to JSONWriter::getDownloader.

(I use the :: syntax to denote instance methods.)