Watchers autocomplete fails with 403 error when the search is made from multiple objects with different projects
Autocomplete is broken when bulk adding watchers for issues from different projects.
How to reproduce:
1. Open /issues.
2. Select two issues from different projects.
3. Press RMB.
4. Watchers => Add.
5. List of users will be shown.
6. Try to filter users.
This is happens because
WatchersController can't find project for
Autocomplete_for_user actions and authorize a user.