Service interruption via backtracking linkifier patterns

Regular expression denial of service in Zulip before 4.7 allows remote authenticated users to interrupt service through linkifiers. The linkifier validator uses a backtracking regular expression to inspect administrator-supplied patterns but accepts arbitrary expressions inside named groups, so both validation and later message matching can consume excessive CPU. Organization-administrator access is required; a malicious pattern or a message that exercises it can deny service without exposing or modifying data.