Rename Course Theme to Learning Mode

[onubrooks]

Fixes #5580

Changes proposed in this Pull Request

• Changes every reference of course theme to learning-mode, this including 'Course Theme', 'Course_Theme' and other variations.
• Changes the course meta _course_theme to _learning_mode as well as a mini migration of the former value.
• Renames all files and directories.
• Renames CSS classes and JS variables.
• Leaves an alias of affected PHP class names.

Testing instructions

This is a big one.

• Test thoroughly all learning mode features. Test enabling and disabling learning mode.
• Run the tests locally to double check: npm run test.