Flexible search endpoint

[kyungmin]

The /search endpoint should allow searching and filtering in the following format:

/search?scope[in]=order,transaction&merchant=CU123&buyer=CU456&status=succeeded
// would return all orders and transactions matching the following filtering params

Maybe we can even allow searching by any arbitrary field:

/[email protected]
// would return all resource types matching the email address

This will also resolve many search-related issues discussed in: #721.

Since the dashboard relies heavily on search to display different associations, it would be nice to achieve this level of flexibility, instead of filing issues for unsupported search params.

Reference: https://github.com/balanced/balanced/issues/717