Users should not be able to edit content hierarchy

[khattam2]

Fixes #1306

Description

What?

Users should not be able to edit content hierarchy

Why?

By editing the parent selection it is possible to create a circular reference loop between two learning objects pointing to each other as a parent, that leads to excessive number of database operations, and as a result, leading the system to become unusable due to heavy database load.

How?

Now the parent field has been disabled so the case should not occur again.

Fixes #1306

Testing

What type of test did you run?

• [ ] Django unit tests.
• [ ] Selenium tests.
• [ ] Other test. (Add a description below)
• [ ] Manual testing.

The parent field in the hierarchy has been disabled. It was tested and is no longer editable as seen in the picture.

Did you test the changes in

• [ ] Chrome

Think of what is affected by these changes and could become broken

Translation

• [ ] Did you modify or add new strings in the user interface? (Read about how to create translation)

Programming style

• [ ] Did you follow our style guides?
• [ ] Did you use Python type hinting in all functions that you added or edited? (type hints for function parameters and return values)

Have you updated the README or other relevant documentation?

• [ ] documents inside the doc directory.
• [ ] README.md.
• [ ] Aplus Manual.
• [ ] Other documentation (mention below which documentation).

Is it Done?

• [ ] Reviewer has finished the code review
• [ ] After the review, the developer has made changes accordingly
• [ ] Customer/Teacher has accepted the implementation of the feature

Clean up your git commit history before submitting the pull request!