Additional optional features added: User management during import

[dasistwas]

Dear @mackensen ,

we did a modernisation of the overall code base and added a new feature: Solve user conflicts upon restore. We implemented it in a way so that when you upgrade, nothing changes. Only if you activate the new features in the plugin settings an admin level, then the new features are activated. New features are:

1. Resolve user conflicts upon restore: Sometimes it happens, that user credentials from remote are slightly different to the ones on the local Moodle. Sometimes that causes the restore to fail. Cases are: Same username, different e-mail, and similar. So we integrated a check and then provide to solve the conflicts: Either merge the user with existing user, or create a new user or just ignore the user and remove the user from the restore process.

2. We added the possibility to choose between username or idnumber for users being matched between local and remote host. Example: If your local admin hast a different username than on the remote, you can now just enter the idnumber (optional user profile fields) so that these match on local and remote. Then you can safely perform the import.

I did a rebase with your changes, so the merge can be done automatically. Tested for Moodle 3.5 to Moodle 3.9. Also travis is happy.

I hope you can merge those changes and if you have any questions, just contact me.

Kind regards,
David