Revert "Mention processing: use \b to match word boundaries."
Revert "Mention processing: use \b to match word boundaries."

This reverts commit 920a4334b44b9ac1893f1218a407dd29d9265bb1.

matrix-org/matrix-appservice-irc,matrix-org/matrix-appservice-irc,matrix-org/matrix-appservice-irc