ROL-1739 add a unique constraint on (website, tagname) to trap a potential error in the code.  Only done for new databases created by createdb.vm; not for old databases upgrading via the migration script.

git-svn-id: https://svn.apache.org/repos/asf/roller/trunk@1615466 13f79535-47bb-0310-9956-ffa450edef68