Pluggable components from datasource plugins

- when instantiating a datasource, the datasource service checks if the
  plugin module exports Explore components, and if so, attaches them to
  the datasource
- Explore component makes all major internal pluggable from a datasource
  `exploreComponents` property
- Moved Prometheus query field to promehteus datasource and registered
  it as an exported Explore component
- Added new Start page for Explore, also exported from the datasource

scripts/webpack/webpack.common.js

@@ -16,7 +16,7 @@ module.exports = {
     publicPath: "public/build/",
   },
   resolve: {
-    extensions: ['.ts', '.tsx', '.es6', '.js', '.json'],
+    extensions: ['.ts', '.tsx', '.es6', '.js', '.json', '.svg'],
     alias: {
     },
     modules: [