Allow consultants & admins to create client accounts

[nhoening]

We should allow consultants, as well as admins, to create accounts in the UI.

• We'd need a simple API endpoint: POST /api/v3_0/accounts. Fields that can be added initially: name (required), primary_color, secondary_color. If current user is a consultant, their account becomes the consultancy account. Look out: we need to provide a schema, so that the endpoint looks good in SwaggerDocs. Also add a test or two (one succeeding, but also some failing for auth reasons).
• The account creation should be a rather straightforward form on a new page, accessible from a button under the /accounts page
• All users with admin role can see the page and use the API endpoint. Users with the "consultant" role should also have the account role "CONSULTANT_WITH_OWN_CLIENTS" (role name up for discussion, and see sketch for implementations of auth structure below).

To use the permission system here, we still need some work, We usually use the "create-children" permission to create new resources, looked up on the parent resource. Accounts have no parent. I propose to trial out creating a virtual "FlexMeasuresPlatform" resource with its own __acl__ - so the API endpoint can use the usual way to check permissions:

@permission_required_for_context(
        "create-children", ctx_loader=FlexMeasuresPlatform.init
    )

Also, the views can use check_access(FlexMeasuresPlatform.init(), "create-children") in a similar way to decide whether to show the button.

And the resource (can live in models/init.py):

FlexMeasuresPlatform(AuthModelMixin):

    def __init__():
        """ Objects don't really do anything but provide the ACL."""
        return self

    def __acl__(self):
        """
        Virtual representation of the platform, to be able to describe create-children permission
        """
        return {
            "create-children": [
                f"role:{ADMIN_ROLE}",
                    (
                         f"role:{CONSULTANT_ROLE}", 
                         "account-role:CONSULTANT_WITH_OWN_CLIENTS"
               ]
           }

[nhoening]

Probably a good idea to make sub-issues (API endpoint, UI form)