Add blog post about open-sourcing the Gitter mobile apps (11-22)

Closes #5906 (closed)

Add blog post about open-sourcing the Gitter mobile apps

Review app: https://eric-gitter-open-source-mobile-apps-blog-post.about.gitlab-review.app/blog/2019/12/01/open-sourcing-the-gitter-mobile-apps/

---

• https://gitlab.com/gitlab-org/gitter/gitter-android-app
  • Internal issue: https://gitlab.com/gitlab-org/gitter/gitter-android-app/issues/1
• https://gitlab.com/gitlab-org/gitter/gitter-ios-app
  • Internal issue: https://gitlab.com/gitlab-org/gitter/gitter-ios-app/issues/1

---

https://about.gitlab.com/handbook/marketing/blog/

---

Checklist for writer

• Link to issue added, and set to close when this MR is merged, #5906 (closed)
• Due date and milestone (if applicable) added for the desired publish date
• If time sensitive
  • Added ~"priority" label
  • Mentioned @rebecca to give her a heads up ASAP
• Blog post file formatted correctly, including any accompanying images
• All relevant frontmatter included
• Review app checked for any formatting issues
• Reviewed by fellow team member
• Assign to @rebecca for review

/label ~"blog post"