There are two different ways an author ID becomes associated with a
user: either bound to a token or bound to a session ID. (The token and
session ID come from the `token` and `sessionID` cookies, or, in the
case of socket.io messages, from the `token` and `sessionID` message
properties.) When `settings.requireSession` is true or the user is
accessing a group pad, the session ID should be used. Otherwise the
token should be used.

Before this change, the `/p/:pad/import` handler was always using the
token, even when `settings.requireSession` was true. This caused the
following error because a different author ID was bound to the token
versus the session ID:

> Unable to import file into ${pad}. Author ${authorID} exists but he
> never contributed to this pad

This bug was reported in issue #4006. PR #4012 worked around the
problem by binding the same author ID to the token as well as the
session ID.

This change does the following:
  * Modifies the import handler to use the session ID to obtain the
    author ID (when appropriate).
  * Expands the documentation for the SecurityManager checkAccess
    function.
  * Removes the workaround from PR #4012.
  * Cleans up the `bin/createUserSession.js` test script.