Generate just an OBF file when exporting a single board

[sylvansson]

This PR changes the behaviour when exporting a single board in the Open Board Format to generate a single OBF file rather than an OBZ archive. I tested this in my browser and everything seems fine, but I'll do another round of QA tomorrow evening. I suggest someone else test this too.

A few notes:

• Images are embedded in the file with the data property. It would be useful to embed sounds as well, but it seemed out of scope so I simply added a TODO.
• OBF files are pretty printed to make them easier to read and edit manually.
• Exports in the Open Board Format are now prefixed with the date, and the time uses the 24-hour clock. This makes exports easier to sort chronologically.

@martinbedouret @shayc Have we considered or tried using TypeScript before? I started using it for this task but it got slightly out of hand, so I undid the changes. It seems we could use the type safety.

Close #653

[martinbedouret]

I have been testing it and worked great. Still need to do testing using symbols from APIs and do some code review.

[martinbedouret]

Great job @sylvansson ! My only comment is to use the same naming convention you are using for obf's, on the other two types (pdf and cboard ).