StudentTasksRequestHandler::applyRecordDelta() should re-use existing StudentTaskSkill records

[themightychris]

The following code block within StudentTasksRequestHandler::applyRecordDelta() will create fresh StudentTaskSkill records each time any changes to the list are saved:

foreach ($skills as $skillId => &$StudentTaskSkill) {
    $StudentTaskSkill = StudentTaskSkill::create([
        'SkillID' => $skillId
    ]);
}

$StudentTask->TaskSkills = array_values($skills);

It should instead have logic to re-use existing StudentTaskSkill records from a map of existing ones by skillId. This will reduce unnecessary churn of database rows and IDs