Datasource for Grafana logging platform

- new builtin datasource plugin "Logging" (likely going to be renamed)
- plugin implements no panel ctrls yet, only ships datasource
- new models for logging data as first class citizen (aside from table
  and time_series model)
- Logs as new view for Explore
- JSON view for development

Testable only against existing logish deployment.
Then test with queries like `{job="..."} regexp`.

public/app/containers/Explore/JSONViewer.tsx

@@ -0,0 +1,9 @@
+import React from 'react';
+
+export default function({ value }) {
+  return (
+    <div>
+      <pre>{JSON.stringify(value, undefined, 2)}</pre>
+    </div>
+  );
+}