Until now in the get_assignments WS all the settings for assignment
plugins (submission and feedback) are returned, this was bad because:

- It was returning settings for plugins that may not be enabled
- It was returning settings that would be a security risk (a third
party plugin including a password or credentials for connecting a
remote site)
- Some values was returned "raw" without further processing, for
example, maxsubmissionsizebytes returned 0 when using the plugin
default value instead a valid size
- Assignment plugins implemented an API for communication with the
external API, see for example: get_external_parameters, it didn’t make
sense had an API for the received parameters and not the returned data.