* time selector for explore section
* mostly ported the angular time selector, but left out the timepicker
(3rd-party angular component)
* can be initialised via url parameters (jump from panels to explore)
* refreshing not implemented for now
* moved the forward/backward nav buttons around the time selector
* moved datasource related functions to panel sub-class
* expand panel template vars for url
* added keybindings for x -> Explore
* url encoding for explore state
* 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
* adds +/- buttons to query rows in the Explore section
* on Run Query all query expressions are submitted
* `generateQueryKey` and `ensureQueries` are helpers to ensure each
query field has a unique key for react.
