text: Title: Preview extension: switch to LabIcon for the refresh button
  Body: The switch to JupyterLab 2.0 was done in https://github.com/voila-dashboards/voila/pull/555.
  
  One noticeable change in 2.x is the way icons are handled. Currently the dark theme is not handled properly (the refresh icon is too dim):
  
  ![image](https://user-images.githubusercontent.com/591645/82885444-7dc76200-9f45-11ea-8f3d-1546addfe6ed.png)
  
  Here is an example PR that implements the switch to `LabIcon`: https://github.com/jupyterlab/debugger/pull/376
  
  For the preview extension, this would mean importing the refresh icon with:
  
  ```typescript
  import { refreshIcon } from '@jupyterlab/ui-components' ;
  ```
  
  And using it here (with `icon` instead of `iconClass`): https://github.com/voila-dashboards/voila/blob/80b120c805fba3f3b33bc5b46f9eea52473b6f78/packages/jupyterlab-voila/src/preview.tsx#L65