[issue_tracker] Enable editing of site, assignee and watchers in Batch Mode

[ay-bh]

Brief summary of changes

• Added site selector to Issue Card edit mode
• Enhanced Add Comment modal with assignee and watchers selectors
• Updated backend endpoint to fetch otherWatchers and assignees

Testing instructions

1. Go to Issue Tracker Batch Mode
2. Click "Edit Issue" on any issue
3. Verify site can be modified using the new dropdown
4. Update Issue
5. Click "Add Comment"
6. Verify assignee and watchers can be modified in the modal
7. Submit changes and verify they persist

Link(s) to related issue(s)

• Part of [issue_tracker] batch mode - follow up #9418

Screenshots