Admin can create Agency Admins & Agency Admins can edit details of their own agency

This will allow the super admin user to create users that act as admins for a specific agency.
The agency admins can then edit and review their agency's details. 
Very nice! Sometimes I will even turn this into a separate method

```
def user_params_with_agency
   user_params.merge(agency_id: params[:agency_id])
end
```

Then use that in the controller action

```
def create
  ...
  @user = sign_up(user_params_with_agency)
  ...
end
```
Your preference. Nice job!