Simplify loader

[obulat]

Fixes

Fixes WordPress/openverse#1712 by @obulat

Description

This PR creates a DbColumn class that defines a database column, and describes its behavior on conflict, when updating data. A list of columns for different media types and loading and main database (openledger, also referred to as upstream) makes it easy to create SQL query strings.
The PR moves the mediatype-specific columns to the end of the loading table, so that the column lists for different media types can be created simply by concatenating a list of common columns with a list of media type - specific columns. This caused a lot of tests to break.
This PR also removes legacy workflows for cleaning up DB tables, used before the ImageStore was used to clean up media data. They are no longer necessary since all the back up data we have has already been cleaned, and the data we collect from the APIs will be cleaned up through ImageStore.

Technical details

Tests

Screenshots

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[dhruvkb]

Thanks for this, keeping media-specific columns at the end is a great idea. From an overview LGTM. I'll review more comprehensively later today.

[obulat]

Closing this in favor of a better-organized #168