Further improvements for the Course module

The course module has all the basics to host a course in the community while remaining lightweight. Here are some feature requests that might be relatively easy to implement and will improve the user experience:
- Complete lesson button at the bottom: It makes more sense that the button to complete a lesson is located where the user will end up after going through the content, instead of forcing them to scroll back up.
- Previous/Next buttons also at the bottom: for the same reason.
- Complete lesson as an available button/block to add, where the course creator chooses.
- The possibility of adding other blocks also in Quizzes: It's currently only possible to add questions, which limits the possibility of adding relevant context, content, and other valuable things.
- Notification when completing a course: currently, the fact that a course has been completed is hidden in the background, not giving enough sense of accomplishment; a pop-up notification with a custom message would elevate that.
- Drip according to completion, not time: as in, unlock the next section (or lesson) after completing the previous one (not in a set number of days afterwards). This is more intuitive than the current options and makes sense in relation to how learning works.