- Drop `explorer-container` and move its logic to `admin-plugin-explorer` container
- Convert resizing of the query edit pane from jquery -> draggable modifier
This is the new version of #126. Now the pick-files-button moved to core. To import a query into Data Explorer, you need to push the button and then deal with the import modal. Instead, we want just to be triggering a system file picker directly. This PR makes it happen.