mail/thunderbird: update to 52.9.0

Changes: https://www.thunderbird.net/en-US/thunderbird/52.9.0/releasenotes/

PR: 229452
Approved by: jbeich (gecko, maintainer)
MFH: 2018Q2
Security: cd81806c-26e7-4d4a-8425-02724a2f48af