Allow custom fields to have custom fields

[colemanw]

Overview

Theoretically a custom field could have custom fields! Trippy...
Fixes dev/core#3731

Before

An extension like https://github.com/eileenmcnaughton/custom_custom won't work, because no one ever imagined we'd need to save custom data for custom fields.

After

Now it works. I still don't understand why anyone would need it, but it works.

[civibot]

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[JoeMurray]

This is how we end up having a core with bloated feature set. I am experiencing a bad trip. ;)