[Bug] Single click on markdown cell selects all text in student mode

### What happened?
Clicking once on a markdown cell selects the entire cell. This makes the cell difficult to read and contradicts default behaviour.

### How can we reproduce this issue?
Open a worksheet in student mode
Click on a markdown cell

### What was supposed to happen?
Nothing. Double clicking the cell is allowed to select the whole cell, but is not strictly necessary.
