Add back inserted_at field to export

[Maxr1998]

This was part of the previous exports and should probably be kept. Untested, but should hopefully work.

I also noticed that the recording_msid is now part of the track_metadata, while previously it was part of both the listen object and the additional_info.

[mayhem]

Could you also please update the test related to this: https://github.com/metabrainz/listenbrainz-server/blob/master/listenbrainz/tests/integration/test_export.py#L168

[Maxr1998]

I'd love to, but it appears that the asserts currently compare to the uploaded JSON, which shouldn't contain insertion timestamps. I don't think I understand the test architecture well enough to handle it. Do you maybe have some pointers on what needs to be done?