Prevent potential misuse of the "email to a friend" feature

Nobody has claimed this yet.

### Description
I would like to point out the potential misuse of the "email to a friend" feature (customers are able to share product links with friends).

The store owner can configure that only logged in users are able to use this feature and something with "Limit sending by: IP Address" (don't know what this does) -- quite well so far. The catch is, one can change the name and the email address of the sender in the form to arbitrary values. In my opinion, that leads to a potential misuse of this feature because one (stalkers etc.) can use this to easily write anonymous messages to their victims.

For sure, registering with an anonymous email and a fake name is always possible but at least these can be identified by the store owner.

### Expected behavior
One can not change the name and the email address of the sender in the "email to a friend" form.

### Benefits
1. I'm well aware that the misuse can not completely prevented in this way. At least It becomes more difficult to misuse this function.
1. Store owners at least have a chance to identify the malicious users.

Start by locating the email-to-a-friend form and the configuration for logged-in users and IP-address sending limits. Review how the sender name and email are accepted, then determine how those fields should be constrained so the sender is identifiable. Done means the form no longer permits arbitrary sender identity values and the existing feature controls still work.

Written by the indexing model from the issue text.