Bug: instructor editing student answer overwrites student authorship

[catinhere]

Summary

When an instructor edits a student's answer, the answer is saved as authored by the instructor instead of the original student.

Steps to Reproduce

1. Log in as an instructor
2. Open a student's answer and click Edit
3. Make any change and save
4. The answer now shows as authored by the instructor, not the original student

Root Cause

Introduced in commit 7f416cb6 ("Fix re-review issues, primarily with self-eval form").

The user_id preset was moved from inside the if ($scope.method == "create") block into the getInstructors async callback in compair/static/modules/answer/answer-module.js, dropping the create-only guard in the process:

// canManageAssignment block — runs for BOTH create and edit
CourseResource.getInstructors({'id': $scope.courseId}, function (ret) {
    $scope.instructors = ret.objects;
    var found = $filter('filter')($scope.instructors, $scope.loggedInUserId, true);
    if (found != "") {
        // preset the user_id if instructors creating new answers  <-- comment says "creating" but runs on edit too
        $scope.answer.user_id = $scope.loggedInUserId;
    }
});

Before 7f416cb6, the preset was correctly guarded inside if ($scope.method == "create"), so editing a student's answer preserved the student as author.

Fix

Wrap the user_id assignment in a $scope.method == 'create' check in compair/static/modules/answer/answer-module.js:

CourseResource.getInstructors({'id': $scope.courseId}, function (ret) {
    $scope.instructors = ret.objects;
    if ($scope.method == 'create') {
        var found = $filter('filter')($scope.instructors, $scope.loggedInUserId, true);
        if (found != "") {
            $scope.answer.user_id = $scope.loggedInUserId;
        }
    }
});