nginx: Update TLS settings based on Mozilla recommendations 5.0.
nginx: Update TLS settings based on Mozilla recommendations 5.0.

Disable TLS 1.0 and TLS 1.1.  (We no longer need to support IE8 on
Windows XP.)

Prefer client-selected cipher order.  (Now that all enabled ciphers
provide good security, this allows mobile clients lacking AES hardware
acceleration to pick ChaCha20 for better performance.)

Disable session tickets.  (Mozilla discourages them based on
https://www.imperialviolet.org/2013/06/27/botchingpfs.html.)

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulipchat.com>

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