WorkItems: Add iteration quick action

What does this MR do and why?

Add work item quick action and autocomplete for iterations.

1. Adds *iteration: autocomplete to work items for description & comments
2. Adds /iteration & /remove_iteration quick actions to work items

Issue: #420790 (closed)

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Go to any work item - you could do this by replacing issues in the URL with work_items or by visiting any epic(provided you have new epics experience enabled)
2. On typing *iteration: in either description or comments should display list of all available iterations.
3. Make sure iteration widget is available for work item , then you should be able to see /iteration and /remove_iteration quick actions. Note - /remove_iteration is only visible if the work item has any iteration attached.