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.