Custom rules for auto verification

[KrzysztofMadejski]

"This field should be verified if entries "

Custom rules examples:

• two points are geographically close
• equals

Distinguish from validation in form input.

[KrzysztofMadejski]

Acceptance Criteria:

• Have a pluggable interface for verification rules
• Write documentation for it, so it's clear how to develop new rules
• Be able to link rules in data model, so it's used automatically for verification

[KrzysztofMadejski]

I'd welcome Levenstein distance for texts as well, but I wonder how such verification should be treated. Even if verified it should go through moderation. Let's drop it for now.