Response `schema` not specified in YAML files

In the Swagger YAML files, response `schema` specs are not given. For example, for `/account_name`:

https://github.com/bitshares/bitshares-explorer-api/blob/9444323252b429fc6b2d9ac2781a05d2f7004c19/swagger/paths_explorer.yaml#L33-L51

In `responses`, it gives only `description` for each status code. The `schema` spec (https://swagger.io/docs/specification/2-0/describing-responses/) is missing.

This may not be a big issue in some use cases. However, we will not be able to use the API with some Swagger clients who rely on the `schema` to unmarshal JSON (like [Bravado](https://github.com/Yelp/bravado)). This makes it harder for non-technical data analysis people to retrieve and make use of the API.

Start with swagger/paths_explorer.yaml and the /account_name responses at lines 33-51, then review the Swagger 2.0 response-schema guidance linked in the issue. Add response schema specifications throughout the Swagger YAML files so clients such as Bravado can unmarshal the documented JSON responses.

Written by the indexing model from the issue text.