Updated foreign key names to be unique

This fixes a bug with MariaDB 10, where webtrees setup fails with the following error:

SQLSTATE[HY000]: General error: 1005 Can't create table `<DATABASE>`.`wt_user_setting` (errno: 121 "Duplicate key on write or update")

This bug is reported in the following places:
https://bugs.launchpad.net/webtrees/+bug/1320605
http://www.webtrees.net/index.php/en/forum/help-for-ver-1-5-3/29283-error-on-install