Designate a single source of truth in JSON specs and autogenerate Python and JS files on build

[rtibbles]

The ground work for doing this has now been laid, updates are needed to accommodate the pre-existing constants and also generate JS code for them.

[ivanistheone]

Yeah would be good to have a single source of truth.

For now I have just added some basic tests to check at least the keys match (assuming well documented in choices):
https://github.com/learningequality/le-utils/blob/master/tests/test_presets.py#L10-L14

note this function should be called test_file_format_preset_keys_are_synced not
test_file_format_extensions_are_synced