Remove @domain.com from username at account creation

[amorrowbellarmine]

This is part of an alternative solution to the request for using the sAMAccountname from AD.
#2157
#2212

This pull requests add an option to use the split UPN for the username during new Moodle account creation rather than the full UPN. This is useful if there is a 3rd party integration that relies on the username to be the sAMAccountname format, as seen in the linked issue.

The usersync code of the o365 plugin already has a function where it splits the user's UPN and matches it to a Moodle username.

o365-moodle/local/o365/classes/feature/usersync/main.php

Lines 991 to 995 in 254fce2

	$upnsplit = explode('@', $upnlower);
	if (!empty($upnsplit[0])) {
	$aadusers[$i]['upnsplit0'] = $upnsplit[0];
	$usernames[] = $upnsplit[0];
	}

o365-moodle/local/o365/classes/feature/usersync/main.php

Lines 1135 to 1149 in 254fce2

	if (!isset($existingusers[$aaduser['upnlower']]) && !isset($existingusers[$aaduser['upnsplit0']]) &&
	!isset($existingusers[$aaduser['convertedupn']])) {
	$this->sync_new_user($aadsync, $aaduser, isset($aadsync['guestsync']));
	} else {
	$existinguser = null;
	if (isset($existingusers[$aaduser['upnlower']])) {
	$existinguser = $existingusers[$aaduser['upnlower']];
	$exactmatch = true;
	} else if (isset($existingusers[$aaduser['upnsplit0']])) {
	$existinguser = $existingusers[$aaduser['upnsplit0']];
	$exactmatch = strlen($aaduser['upnsplit0']) >= $switchauthminupnsplit0;
	} else if (isset($existingusers[$aaduser['convertedupn']])) {
	$existinguser = $existingusers[$aaduser['convertedupn']];
	$exactmatch = true;
	}