Using YAML as a HTTP request body instead of JSON for workflows/workbooks/actions

Now if we create new workbook/workflow/action we send a request with a body like:

{"definition": "escaped YAML"}

It is problematic when we, for example, use command line tools like "curl" to work with REST endpoints directly because we have to:
1. Take YAML definition and convert it to escaped JSON compatible string (not even clear how to do it in a simple way)
2. Create that JSON structure and put that huge escaped string into it.
3. Send a request using providing that JSON file

It would be much simple just specify YAML file in the first place. For this to work we need to modify controllers and corresponding client code for workbooks, workflows and actions.