Fix first line markdown helper for user profile activity stream

What does this MR do?

• Allow gl-emoji in sanitized events
• Safely check for a Nokogiri element description which is nil for unknown elements, http://www.rubydoc.info/github/sparklemotion/nokogiri/Nokogiri/XML/Node#description-instance_method

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

• 500s on user profile page if an activity stream item contained an emoji

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #29425 (closed)