Add param validations to API endpoints

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Each endpoint should have a well-defined set of params and validations on those params. Validations should:

• protect the endpoints from receiving classes of bad or unexpected data
• be easy to declare and test
• should not be coupled to other objects
• should be lightweight

Typical validations include:

• should not be empty
• should be within a range
• should be shorter than a given length
• is numerically positive/negative/zero
• contains a certain number of items

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Proposal: https://github.com/OpenBazaar/openbazaar-go/compare/validation-pattern?expand=1