Phase 1: Add `/gitlab incident timeline <text>` Slack slash command

Problem to solve

Managing incidents via Slack should offer a consistent experience, which means users should expect to update incidents using the incident keyword in Slack slash commands. This also goes for adding new timeline events to the incident timeline.

We want all incident-based slash commands in Slack to require users to remember as little as possible, so users should be able to close a recent incident without remembering the id or project alias.

Proposal

1. Add a /gitlab incident timeline Slack slash command
2. When a user uses the command, the following should happen:
   • user is presented with an "ephemeral" message containing the last 5 incidents created in projects the Slack user has access to, with buttons to add a timeline event to the associated incident
   • user is presented with links to the incident lists for each of the other projects in case their incident isn't in the last 5
   • if the user doesn't have permission to view/add timeline events to any incidents, they should see an error message? [DO WE NEED AN EMPTY STATE?]
3. When the user selects a 'Add event' button, the following should happen:
   • the timeline event is created for that incident, using the current time for when the event occurred
   • the incident-list message should be replaced with a confirmation/error message
     • the confirmation message should indicate the event was created successfully, with a link to the incident
     • if an error occurs during creation, the user should see an informative explanation & link to documentation

Timeline slash command	User prompt to select incident	Success/failure messages

Figma