A teacher can add description to database activity presets

Setup

1. Log in as a teacher and go to a course.
2. Add a database activity (with the default configuration).
3. Create one field (it can be of any type and have any name).

Testing instructions

1. Access the database activity and go to the "Presets" tab.
2. Click the "Export preset" option from the Actions menu.
3. Check a .zip file is downloaded.
4. Extract the .zip file and check it contains several files (a few template HTMLs, CSS, JS...). 
5. Open preset.xml.
6. Check it contains a description element (which should be empty).