TLS deprecation blog post for 10-15 release

Review app: https://2018-1015-blog-post-tls-deprecation.about.gitlab.com/2018/10/15/gitlab-to-deprecate-older-tls/

Hi @erica Thank so much for your help.

Please review and advise.

This merge request is for the TLS Deprecation blog post to be released on Monday 10/15/2018

/cc @kathyw