Add Global Variables

**Why is this needed**: Create global variables that are available on a global level, every single dashboard would have access to this global variables. They could be toggled on and off for individual dashboards. It would solve problems where multiple dashboards are required to link to same continuous custom static data, goal is to map azure queries data variables into a collection. If you need a country variable mapping custom variable with multiple values, with multiple dashboards I need to manually copy the values throughout multiple dashboards when the values are updated. If I had access to global variables, this whole tedious process wouldn't be required. **What would you like to be added**: Add new "Global Variables" tab on dashboard settings. Where all the global variables can be managed / and activated for that specific dashboard. These would be shared globally across all dashboards in grafana. Whenever a new global variable is added, it's turned off for all dashboards until it's activated manually. **Who is this feature for?** All users