Support block on activity pages

[dlmsr]

🔀 Purpose of this PR:

• Fixes a bug
• Updates for a new Moodle version
• Adds a new feature of functionality
• Improves or enhances existing features
• Refactoring: restructures code for better performance or maintainability
• Testing: add missing or improve existing tests
• Miscellaneous: code cleaning (without functional changes), documentation, configuration, ...

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📝 Description:

This implements #62: The block can now be placed on activity pages and generates a QR code which links to these pages.

[dlmsr]

There were two problems with the previous version:

1. The QR code still had the course link instead of the link to the activity page on activity pages.
2. The filename of the image was only dependent on the course id. Therefore, when adding the block to the course page and the activity page, the two blocks showed the same QR code.