Show teacher's questions only when using category questions

[gikaragia]

Overview

In the quiz editor, when a category question was added, all questions were included regardless if they belong to the teacher or not. With this change this is now fixed.

Changes proposed in this Pull Request

• If the quiz author is not admin, then only the questions that he owns are displayed when a category question is expanded.
• If the quiz author is an admin all questions are added since as an admin he has access to all questions.
• In the quiz editor, the question counts have been updated in order for the warning to displayed correctly.
• While I was searching the code, I also did some cleanup.
• @donnapep since this is going to reduce the questions that a quiz has in some of the cases, we might want to mention it in the next release post. Users can use the sensei_filter_category_questions_by_author to restore the previous behaviour.

Testing instructions

• With a teacher, create some questions and add them to a category.
• Do the same with another teacher or with an admin.
• Login with the first teacher and try adding a question category to a lesson's quiz.
• Observe that only the questions that the teacher owns are taken into account.
• Observe in the frontend and in grading that the correct questions are included.
• Repeat the same with an admin and observe that all questions are included.

New Filter

• sensei_filter_category_questions_by_author - Whether category questions should be filtered by author.

Deprecated Functions

• Sensei_Frontend::sensei_get_user_quiz_answers - Use Sensei_Quiz::get_user_answers instead.
• Sensei_Quiz::stop_quiz_questions_loop - No replacement.

[yscik]

I wonder if it's a legit use case that teachers want to share questions with each other. It works currently, right? Would it be safer to keep allowing it by default, and have the filter (or possibly a setting) to disable it?

[jom]

Tested this and it works really well! I also tried switching to this branch when the learner has a saved quiz but hasn't submitted it and it doesn't remove the questions (which is good, I think).

One tiny doc change and then I'm ready to approve.

@yscik Before teachers weren't allowed to add category questions (just if an admin transferred an admin course to a teacher). I think it is safer to assume not so other teacher's questions don't start appearing on their quizzes.

[gikaragia]

@yscik just wanted to add that I think that the main issue with the current way this works is that while questions belonging to another teacher can be included in the quiz, the quiz owner cannot access these questions in admin.