This changes the capability computation from code (a very large if
statement) to transformation of static data.

The data we transform is a map from `ability` to `requirements`:

```ruby
{
  ability_needed: [:test_one?, :test_two?],
  other_ability: [:test_three?]
}
```

If the object passes _any_ of the requirement tests, then it must pass
the policy ability check for all of the matched abilities. If the tests
are inductive then this becomes a one-to-one reverse mapping, but it
need not be, so this formulation can actually express stricter policy
tests than the current if statement can (i.e. we can assert that some
event needs multiple abilities).

This change expresses (like a DSL) the rules as symbols that must be
implemented as methods on the receiver, so we need to `send` the
requirements to the receiving event.