Blog post for 9.3.5 patch release

Review app url: https://add-9-3-5-blog-post.about.gitlab.com/2017/07/05/gitlab-9-dot-3-dot-5-released/

Checklist from @rymai's suggestions in !6634 (merged):

• Changelog entries added for CE, EE & Omnibus
• Downtime message updated
• Links verified
• Changlog entries have consistent style for
  • Verb tense Fix, Fixing or Fixed
  • Full-stop placement (none, before link, or after link)
  • Use of backticks to escape versions, code and projects
• Lines wrapped to 80 chars