Enable to sort challenges by createdAt and updatedAt

[tschaffter]

Fixes #123

Notes

• In the challenge list path, I added two query parameters named sort and direction using the same parameter name and schema as in GitHub API. This information will be used by the API service to sort results before returning paginated responses.
• With this PR, I propose to swap the parameter properties name and in so that name comes first. This is because one is probably more interested in the parameter name, then the type of the parameter. This order is also more consistent with traditional schema where each property have their name on the first line. Example:

Before:

    - in: query
      name: limit
      description: Maximum number of results returned
      required: false
      schema:
        type: integer
        default: 10
        minimum: 10
        maximum: 100

After:

    - name: limit
      in: query
      description: Maximum number of results returned
      required: false
      schema:
        type: integer
        default: 10
        minimum: 10
        maximum: 100

If this proposal is accepted, this change will be propagated to all other path parameters in a separate PR.

[vpchung]

I propose to swap the parameter properties name and in so that name comes first

Sounds good to me. I think Swagger lists them like this as well.

[tschaffter]

@vpchung Swagger doc didn't adopt a consistent behavior (example: https://swagger.io/specification/)

[vpchung]

lol well that's awesome....