Rewrite of adding custom data to custom DC

[kmoscoe]

This PR is a complete rewrite of the "Load custom data" page, including:

• Distinguish between "implicit" and "explicit" schema
• Add instructions on how to choose between both
• Add description of variable per row, including MCF and config.json fields
• Add CSV, MCF and config.json examples based loosely on UN-WHO data
• Make the config.json section be a reference to all available fields

It does not yet include topics or custom entities (coming next).

Staged at bullie.svl.corp.google.com:4000