Move to (good) single-database mode
Having several databases have killed any attempt at having an easy installation process and have misleaded tons of people into believing that the current single-database mode is a better idea. This has to change.
The change to one single database implies unifying tables and adding a course_id field in every one of them (see #1244). This is a highly risky process, and automated tests should be put in place before development to ensure we don't loose features.