Work item quick actions: add_email and remove_email and external participants feature

Problem

We need to support all quick actions currently available on legacy issues.

Proposal

• Add support for /add_email quick action that behaves exactly the same as on legacy issues (https://docs.gitlab.com/ee/user/project/service_desk/external_participants.html#add-an-external-participant)
• Add support for /remove_email quick action that behaves exactly the same as on legacy issues (https://docs.gitlab.com/ee/user/project/service_desk/external_participants.html#remove-an-external-participant)
• This also involves adding all missing external participants functionality (part of Service Desk feature)
  • Correctly display notes from external participants
  • Display the warning banner when a notification email would also go to external participants
  • Display the warning banner when attachments are >10MBs and external participants are present
  • Emails properly sent for new public notes
  • Emails are properly sent when external participant was added

Implementation details

See this note - #462118 (comment 1909067996)