Currently, the button "Send Email" next to the email address of an event
registration didn't work if a partner was not set.
This commit removes the button, thus removing the feature but partially
adds it back via the chatter suggested recipients in the same way it is
done for a lead.
It is still partially a feature regression since to work the viewing
user need more access rights (e.g read access right on res.partner)
than before.
closes#9486
opw-653127
9717afbbb7