Improve configuration of accounts administrators

In the accounts API, the setup of administrators requires two steps:
* creating the actual accounts
* adding the principals in the settings

Instead we could have a `kinto createadmin` command for example.

Start by tracing the accounts API's current two-step administrator setup and the existing `kinto` command entry point. Define what the proposed command should accept and how completion is verified: creating the accounts and adding their principals in settings as one administrator-configuration workflow.

Written by the indexing model from the issue text.