Lectures: show appeal date

For an absence an appeal can be handed in. This can be done several times, as the email can be send again and again. further there is no information, that the appeal has already been sent. 

To do

• Appeal email can only be sent once
• in the row "Appeal" of the user view, it should not only be written appeal, but e.g. also the date, when the email has been sent. This only after the email has been sent.