Hotkeys: Make time range permanent (#43802)

Typing `t a` in Explore or Dashboards will turn a relative time like "Last 1 hour"
into an absolute range to make the URL permanent, so that when sharing it others
will see the same data.

- registered `t a` in key service
- new `AbsoluteTimeEvent` dispatch via global event bus
- dashboard times handled in TimeSrv
- Explore times handled in Explore.tsx and Explore's time reducer

I could not find an easy way to combine time handling for Exlore and Dashboard in one place.

public/app/types/events.ts

@@ -144,6 +144,10 @@ export class ShiftTimeEvent extends BusEventWithPayload<ShiftTimeEventPayload> {
   static type = 'shift-time';
 }
 
+export class AbsoluteTimeEvent extends BusEventBase {
+  static type = 'absolute-time';
+}
+
 export class RemovePanelEvent extends BusEventWithPayload<number> {
   static type = 'remove-panel';
 }