Associate a work item to a milestone
## Problem
There is no way to set a milestone on an issue. We can view a list of possible milestones now, but there is no way to save the selected milestone.
## Proposal
- When viewing a list of possible milestones, I can select one of the milestones from the list to associate an iteration with a work item.
- If the work item already has a synced milestone, the manually selected milestone takes precedence and the "sync" is disabled.
## Acceptance
- [x] Associate a work item to a milestone.
- [ ] Properly handle disabling the sync and allowing the manually set milestone to take precedence.
- [x] Redis counter
## Redis Counter
* **name:** `users_updating_work_item_milestone`
* **category:** `work_items`
* **redis_slot:** `users`
* **include in the following aggregation counters:**
* [x] `xmau_plan`
* [x] `xmau_project_management`
* [ ] `xmau_product_planning`
* [ ] `xmau_certify`
* [x] `users_work_items`
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