text: Title: Convert google slide URLs to PDFs (as with powerpoint etc)
  Body: **Is your feature request related to a problem? Please describe.**
  People increasingly add presentations as links to google slides. This is bad  as these URLs can expire/be deleted in which case the content is lost. Of course, we can ask speakers to remember to also update a pdf version, but since indico can autoconvert other file formats, it would be great if it could do it for google slide URLs as well.
  
  **Describe the solution you'd like**
  
  Use google API to convert to pdf
  ```
  GET https://www.googleapis.com/drive/v3/files/{fileId}/export
  ```
  as described [here](https://developers.google.com/drive/api/reference/rest/v3/files/export).
  
  where the `{field}` can be extracted from a URL e.g.
  ```
  https://docs.google.com/presentation/d/1kbmidlabdSPHUAgS2sZOCGaMqvCmjSM6Kk2p9LSH3Oo/edit#slide=id.p
  ```
  and the `mimeType` is obviously `application/pdf` e.g. using the URL above:
  
  ```
  https://developers.google.com/drive/api/reference/rest/v3/files/export?apix_params=%7B%22fileId%22%3A%221kbmidlabdSPHUAgS2sZOCGaMqvCmjSM6Kk2p9LSH3Oo%22%2C%22mimeType%22%3A%22application%2Fpdf%22%7D
  ```
  