'Del bog' feature

[NajaWulff]

Testing the this feature might require setting up accounts for all the different clients to see if the mails received are actually working.

---- :: Conclusion :: ----
This feature seems to work without issues. One remark: When trying to send a link to a non-free book to someone who then logs in as a guest user, the guest is simply logged in to the default guest book shelf - the fact that the book from the original link is not there is not made explicit to him/her.