Resolves https://github.com/Automattic/sensei-security/issues/24

Proposed Changes

• Use wp_strip_all_tags instead of a discouraged strip_tags

Testing Instructions

0. Not all the changes is possible to test on a working website. Please, inspect the changes.
1. Go to Sensei LMS -> Questions.
   • Add a question with arbitrary data.
   • Go to DB and in {prefix}_terms table and change the name of the term to have tags, for example: UPDATE sensei_test.wp_termsSETname= 'multiple-<b>choice</b>',slug= 'multiple-<b>choice</b>' WHEREterm_id = 13; (Make sure the resulting string after stripping is the same as the original, so it shouldn't be something like multiple-choice<b>123</b>.)
   • Go to Sensei LMS -> Questions and make sure you see the type for your question (should be "Multiple Choice", not "—").
   • Revert changes in DB.
2. Go to Sensei LMS -> Questions -> Question Categories.
   • Add a new category.
   • Assign the category to your question.
   • Go to DB and adjust the category name to have tags. Something like: UPDATE sensei_test.wp_termsSETname= '<b>test1</b>' WHEREterm_id = 14;
   • Go to Sensei LMS -> Questions and make sure you see the category in the list for your question without tags.
   • Don't revert these changes in DB for now.
3. Install and enable Classic Editor.
   • Go to Sensei LMS -> Lessons.
   • Create a new lesson.
   • Find Quiz Questions panel.
   • Click on the "Existing Questions" tab.
   • It is expected to see tags in the category dropdown, but not in the category column.
4. Activate a theme that supports widgets. For example, Twenty Twenty-One.
5. Go to Appearance -> Widgets.
6. It's not possible to test two remaining changes in Sensei_Admin (we don't have settings there that use textarea) and in Sensei_Settings (we strip tags from escaped lines).

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• Code is tested on the minimum supported PHP and WordPress versions