Fix classlist exporting with users that do not have passwords.

[drgrice1]

The exportUsersToCSV method of WeBWorK::ContentGenerator::Instructor::UserList assumes that all users have password and permission records in the database. Since that is no longer the case, that code is not working right. In fact if some users do not have password records and others do, then the resulting classlist that is exported will have many passwords in associated to the wrong users. Some of the users that don't have passwords will have passwords, and vice versa. Note that the code had defined statements checking if the permission and password records exist, which mean that the code was technically incorrect to begin with. It only worked because usually all users had password and permission records.

The general issue with this is noted in #2704 with regards to the deletion of the code that auto creates password or permission records when getPasswords or getPermissionLevels is called. The general issue is that the getPasswords and getPermissionLevels methods call the WeBWorK::DB::Schema::NewSQL::Std::gets method which does not even include non-existent records. So for example, if getPasswords is called with the list of user IDs 'user1', 'user2', 'user3' and user2 does not have a password, but the others do, then getPasswords will return an array with two elements which are the password records for user1 and user3. So when iterating by index on the original list the password record for user1 will correctly go to user1, but the password record for user3 will be paired with user2, and user3 will not have a password record.

Also, the exportUsersToCSV dies if the provided filename contains a forward slash. So now the JavaScript form validation checks for this, and prevents those filenames from being submitted to the server. Thus preventing the die statement from occurring.

[pstaabp]

Tested that the code in develop does not export passwords correctly and that this does fix the issue.