LB-1419: Fix broken partial dates

[mayhem]

The MB data cashes have invalid partial dates for release_group data. The problem is that when any part of a || concat expression in PG is NULL, the entire result is NULL. The solution is to move the partial data logic into python code.

[pep8speaks]

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

• In the file mbid_mapping/mapping/mb_artist_metadata_cache.py:

Line 83:131: E501 line too long (190 > 130 characters)

• In the file mbid_mapping/mapping/mb_release_group_cache.py:

Line 137:9: E303 too many blank lines (2)

[mayhem]

This PR is also temporarily deployed, anticipating the monthly rebuild of all of the data.

[amCap1712]

I am not sure we are relying on the date to be parsable anywhere but in case something breaks we should keep this change in mind. Also, you could use COALESCE in postgres to generate the partial dates but python is fine too.