all: pass errors to panic, not strings

GraphQL's recovery middleware can't handle strings so it just logs a
very not useful <nil>

valid: Add Error function

Add an Error function which is similar to Errorf except that it does not
take a format specifier.

valid: Add NullableString

Add a NullableString method to allow users to differentiate between the
absence of a field and an explicitly provided null value.

valid: Add Errorf function

Add an Errorf function to create GraphQL errors without a validation
context.

valid: add OptionalBool

valid: fix nil values in OptionalString

valid: add generic optional interface

auth: force webhooks to read-only

valid: accept format strings

valid: Fix Sprintf call

valid: new module for input validation