FlaskPlugin: use get_id() method instead of id attribute

[jirikuncar]

Following Flask-Login documentation we should use current_user.get_id() instead of current_user.id. See https://flask-login.readthedocs.io/en/latest/#your-user-class

[anthraxx]

@marksteward @jirikuncar This is actually a misinterpretation of the API, this commit should be reverted:

The API expects get_id() to be a unique string, that may or may not be an integer. This feature is only used to uniquely identify the user, but its not the same as the user's primary key.

Please take a note on how get_id() is expected to behave on this feature of flask-login:
https://flask-login.readthedocs.io/en/latest/#alternative-tokens
It means it can return any arbitrary string to identify the user which can be swapped to any other value to invalidate all sessions. This explicitly states that it is not the same as the user's primary id which is also used as foreign key on the table.

sqlalchemy-continuum's transaction tables reference the user with a foreign key of the actual id. If a downstream application uses the alternative-tokens feature as described by the flask-login documentation, this breaks horribly as sqlalchemy-continuum will then try to insert an arbitrary unique string of the user as the foreign key to reference the user in the user table.

[marksteward]

Thanks, yes that does look wrong. I misunderstood the context it was being called in - we shouldn't need to call get_id because we're the implementer.