Unmaintained User Mentions Improvements by Xon 1.7.0

• Scrub all mentions of "User Tagging" in favour of "User Mentions".
• Cleanup email template used for user mentions.
• Cleanup installer code.

Occasionally user mentions which where too long would error.

New permission:

• Disable User Mentions

Allows sending of user mentions for specific user groups (globally or per node) to be disabled.

Thanks @laim W for pointing out this resource is confusingly named with the new content tagging system