Replace all occurences of PIL/Python Imaging Library by Pillow

As of 5.0.0, we start using Pillow as the default imaging library for Python.
This commit makes the corresponding changes under the existing documentation
pages.
Add section to server upgrade page about Pillow/PIL
Fix Pillow downloads page URL
Replace list of packages by see also link to server installation
Merge pull request #653 from sbesson/pillow

Update documentation to use Pillow instead of PIL