* panel container menu gets new Explore entry (between Edit and Share)
* entry only shows if datasource has `supportsExplore` set to true (set
for Prometheus only for now)
* click on Explore entry changes url to `/explore/state` via location provider
* `state` is a JSON representation of the panel queries
* datasources implement `getExploreState()` how to turn a panel config into explore initial
state
* Explore can parse the state and initialize its query expressions
* ReactContainer now forwards route parameters as props to component
* `pluginlist` and `singlestat` panel subclasses needed to be adapted because
`panel_ctrl` now has the location provider as a property already
