Autocomplete

This PR adds django-autocomplete-light as a dependency. This enables autocmplete for the users, layers maps and documents in many forms including metadata and in the admin.

It is my intention to remove all other autocomplete implementations including the use of select2 in our permissions form and in the document upload form. These will come with subsequent PRs.

There are a few outstanding issues. For some reason, overriding the widgets in the documents app does not work the same as the Layers and Maps. If we hook up autocomplete in documents/forms.py like it is in layers/forms.py and maps/forms.py an error (https://dpaste.de/n2N2)  is thrown. This is the similar line in layers/forms.py https://github.com/jj0hns0n/geonode/commit/9b20a8893f700c8d9a415ce9fbf764016c4d9f5a#diff-d72a1da91b74bd369f1b5f178877a80dR71 @simod could you potentially look at this? I've left it disconnected for now.