Update issue description when task title/state changes

What does this MR do and why?

Update issue description after changing task title or state in the task modal. Use workItemTaskUpdate, which flushes the cached description of the parent issue.

Move mutations to the work item widgets, to make them more self-contained . Have to pass down workItemParentId to determine which update mutation to use, so made a shared function for that. Maybe it should be a Vue mixin I don't know.

How to test

1. Enable work_items feature flag
2. Create (or edit) an issue. Add a checklist item to the description
3. Hover the checklist item > Create work item
4. Create the work item and click the title to edit
5. Change title or status
6. See title of the task change in issue description after next realtime_changes update

Screen_Recording_2022-06-03_at_11.18.48_am