ResourceCodeService

We need a ResourceCodeService that will generate a new, unique, and child-friendly resource code for a new Resource.

The service should have a public method, new_code_for, that receives a not-yet-saved resource as a parameter, and returns a suitable new code value.

The resource's attributes will dictate what the code should look like:

• presence of an institution_id value should result in a code that includes the institution's code
• presence of a parent_id should include the parent resource's code

And finally, the generated code must be unique.