Bug 1734169: Allow the user to say why he refuses the privacy

Add a textare in the 'are you sure' modal where the
user can write the reason why he refuses to consent to
a privacy statement.
This reason will be sent in a message to the institution
or site admin.

behatnotneeded

Change-Id: I6abe4c8c7c517b1319139497bedb40525d095fcb