fix: init_es.py script not completing due to AttributeError

[vinmh]

This PR fixes a small bug in the querybook.server.logic.elasticsearch._bulk_insert_users() function. The get_users_iter function used to return a dict object representation of the users in the database, but the _bulk_insert_users function tries to access the is_group attribute found in the User SQLAlchemy model.

Running this function with users already present in the database (usually through the init_es.py script invoked by --initdb) results in the following error:

Traceback (most recent call last):
  File “<stdin>“, line 1, in <module>
  File “/opt/querybook/querybook/server/logic/elasticsearch.py”, line 725, in _bulk_insert_users
    if not user.is_group:
AttributeError: ‘dict’ object has no attribute ‘is_group’

The proposed fix is to return User instances from the get_users_iter function and then convert them to ElasticSearch compatible dict objects in the functions that need it (_bulk_insert_users() and _bulk_update_users(fields: Set[str] = None)).