Service Desk: Remove external email participants to service desk via quick action

Allow multiple email participants (added via /invite_email quick action) to be removed via quick action (perhaps /remove_email)

• Only project members (with guest+ access?)
• Preview the action (see designs in #195525 (designs)) with errors if the action can't be done (e.g. not subscribed)
• Add a system note
• No notification to the removed party that they have been unsubscribed
• Remove email_participant row from the db

Use feature flag issue_email_participants

Implementation Plan

The quick action handler will go in lib/gitlab/quick_actions/issue_actions.rb.

Refer to the command :invite_email to get a feel for how it will roughly look.