[security] Protection for exposed emails

## Problem

Plaintext emails are exposed on users' profile pages.  Spam bots may easily scrape for these emails and add them to annoying mass mailing lists.  (I learned this the hard way in 2008 when I used e107 v0.7).

## Solution

In this pull request, there is code for the unobtrusive obfuscation of emails.

## Caveats

`{USER_EMAIL_LINK}` is no longer used in the entire system.  I don't really know what it does... :confused: