Can't add annotations with the keyboard

[javierm]

Steps to reproduce

1. Go to the admin section
2. Create a legislation process enabling the comments phase
3. Create a draft version for that legislation process
4. Visit the page of that draft version (example in the demo)
5. Try to select text with the keyboard and add an annotation

Current Behavior

It's impossible to add an annotation using the keyboard

Expected Behavior

After selecting some text with the keyboard, the button which makes it possible to add an annotation is shown and people using screen readers are notified about the presence of that button.

Notes

We've actually got a test failing sometimes due to this issue (see issue #4713).