Downgrade db schema via Curio Toolkit

[snadrus]

Allow reverting, where authors authorize it.
This provides an avenue for "buyer's remorse" where upgrades with migrations turn out badly.

revert/* authors:

• Since db sql is reentrant, no need to delete tables.
• Migrations MUST migrate back
• SQL INSERTs: are they defaults or do they trigger something?
• Note that the db gets fully upgraded TO THIS RELEASE first.
• not every SQL needs a revert, but all should indicate if they're able to revert.

[magik6k]

Looks good, maybe would consider instructing users to make a backup, they are likely already recovering from a disaster so best to add some guardrails