Fix rendering emoji inside a string

Closed username-removed-86853 requested to merge blackst0ne/gitlab-ce:fix_emoji_parser into master

What does this MR do?

  1. It does not let emojis be rendered if they are surrounded by numbers (0-9), colon (:) or letters ([:alnum:]).
  2. It lets emojis be rendered in all other cases.

Screenshots (if relevant)

Before After
Screenshot_20170412_202243 Screenshot_20170412_202231
Write Preview
Screenshot_20170419_214413 Screenshot_20170419_214934

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #26433 (closed)