Derek furst/constraints allow non donors

[DerekFurstPitt]

Added a bypass inside metadata_constraints.py in the get_constraints function. The original plan was to do the bypass directly in app.py in the endpoint, but because we need to go a few layers down into the request body to find the entity-type, and both that and the part that sets the response code are located there, it worked out better doing the change here.

now donors will be validated as before (donor must have an organ descendent) but non donors will pass automatically with a "nothing to validate" response like we do elsewhere.

[yuanzhou]

@DerekFurstPitt a few things:

• Since now you are handling the bypass validation in metadata_constraints.py, it makes more sense to also move constraints_json_is_valid() to this module so we keep everything in one module.

• Due to report_type is no longer needed for HuBMAP, we should remove the unused index variable too.

• Looks like you are treating all the non-200 codes as 400 error , then what's the point of returning 404 in get_constraints()?

• Within get_constraints(), the result dict has inconsistent use of name and description. We can get rid of name field and only return code and description otherwise, only need to handle name as part of the endpoint response. Really depends on how you want to handle the final response in app.py