Work Item - support quick actions through editing description

Given that we plan on having work item types configured to specific widgets, that means quick actions should also account for the available widgets on a given work item type.

This to consider:

• quick actions autocomplete should be aware of widgets available for a given work item
• quick actions passed in a comment should be validated as applicable on the work item based on available widgets
• quick actions used in a template that can be applied to a work item that does not support all the quick actions in the template

Is there a way to have the widget somehow self register its quick action(s) on given work item type or do we have to fetch available quick actions for a given work item type based on available widgets and check against that?

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This issue will track the implementations of the quick actions that can be applied to existing work items via the description widget.

• Task 1: Support the initial batch of quick actions in the backend

  • Common quick actions :title, :reopen, :close, :tableflip, :shrug
  • Labels widget: :label, :labels, :relabel, :remove_label, :unlabel
  • Assignees widget: :assign, :unassign, :reassign
  • Start and Due Date widget: :due, :remove_due_date
  • Health status: :health_status, :clear_health_status
  • Weight: :weight, :clear_weight

• Task 2: Update the frontend components (enable the quick action and correct the preview markdown path) and update the feature specs.

• Task 3: Support the rest of the quick actions (iteration, milestone and etc.) #389450 (closed)

Release Notes

Quick actions are helpful for efficiently updating issues, merge requests, and epics. With this release, you can now use a subset of quick actions when editing a task's description. Future iterations will introduce additional quick actions and the ability to use them within the context of comments and threads in a task.

https://docs.gitlab.com/ee/user/project/quick_actions.html#work-items