As discussed in a UX feedback session, it's annoying that queries are
automatically executed in Explore. This change adds props to override
the blur behavior.

- add `onBlur` to Explore query field props
- Explore's query row will pass down an empty function for onBlur to the
query fields
- pass onBlur through to the QueryField component for Loki and
Prometheus
- add test to QueryField to make sure if onBlur is specified, the
onRunQuery is not executed