add is_deleted for user in db

backend/README.md

 
     tracimcli db init
 
+Stamp current version of database to last (useful for migration):
+
+    alembic -c development.ini stamp head
+
 create wsgidav configuration file for webdav:
 
     cp wsgidav.conf.sample wsgidav.conf

backend/doc/migration.md

 
     alembic -c development.ini current
 
+## Set Alembic stamp to last version (first time use) ##
+
+    alembic -c development.ini stamp head
+
 ### Creating new schema migration ###
 
 This creates a new auto-generated python migration file 

backend/tracim_backend/models/auth.py

 from hashlib import sha256
 from typing import TYPE_CHECKING
 
+import sqlalchemy
 from sqlalchemy import Column
 from sqlalchemy import ForeignKey
 from sqlalchemy import Sequence
     _password = Column('password', Unicode(128))
     created = Column(DateTime, default=datetime.utcnow)
     is_active = Column(Boolean, default=True, nullable=False)
+    is_deleted = Column(Boolean, default=False, nullable=False, server_default=sqlalchemy.sql.expression.literal(False))
     imported_from = Column(Unicode(32), nullable=True)
     timezone = Column(Unicode(255), nullable=False, server_default='')
     # TODO - G.M - 04-04-2018 - [auth] Check if this is already needed