Add opt-in checkbox for saving timezone changes on dashboards

**Why is this needed** Dashboards default to a configured timezone, but users can temporarily switch to a different timezone for their own view. When a user saves the dashboard after making changes, their personal timezone preference is saved as the new default, overriding the dashboard’s original setting for everyone. A single accidental save can cause confusion to users that rely on consistent timezones. **What would you like to be added** An additional checkbox when saving a dashboard that allows users to opt-in to updating timezone since it's a dashboard wide setting similar to refresh, variables, and timeframe which already have opt-in checkboxes. **Who is this feature for?** All users of dashboards who have different timezone preferences.