LB-1687: Add mapping info to listen exports

[amCap1712]

The mapping information is available in the listens api, make it available in listen exports as well.

[pep8speaks]

Hello @amCap1712! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file listenbrainz/background/export.py:

Line 84:131: E501 line too long (169 > 130 characters)
Line 90:51: W291 trailing whitespace
Line 107:41: W291 trailing whitespace

• In the file listenbrainz/tests/integration/test_export.py:

Line 86:131: E501 line too long (152 > 130 characters)