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)

mentioned in issue #29425 (closed)

assigned to @MadLittleMods

resolved all discussions

assigned to @jamedjo

added 33 commits

• 53aff128...ffcddb29 - 32 commits from branch master
• 3e29936a - Fix first line markdown helper for user profile activity stream

Compare with previous version

Approved this merge request

Looks good to me

@rspeicher @twk3 Is this something we'll want in RC1?

assigned to @rspeicher

@jamedjo yes, otherwise most of our activity feeds will be broken

enabled an automatic merge when the pipeline for 3e29936a succeeds

merged

mentioned in commit 2e051060

mentioned in commit 068fe615

marked the task All builds are passing as completed

This is already picked into 9-0-stable

removed ~149423 label