Add missing initialization of existing_org in organizations selector.

Without this initialization the organizations selector doesn't select the current organization.
[#1199] Make sure we can remove datasets from organizations through the API. The problem was that the validator removed the owner_org from the data dict when it was empty.
[#1199] Upgrade select2 to make it possible to select item with empty value.
[#1199] Fix the package_update and package_create validator to accept an empty org but not None.
[#1199] Change 'Select an organization' to 'No organization'

When creating or editing a dataset, the dropdown for selecting the
dataset's organization contained an item labelled
'Select an organization...' that, if selected, would set the dataset to
belong to no organization (removing it from any organization it does
belong to). This seems quite confusing, change the label to
'No organization' to make it clear what it does.
Merge branch 'master' of github.com:okfn/ckan into 1199-fix-package-owner-org-edit
Merge branch 'master' of github.com:okfn/ckan into 1199-fix-package-owner-org-edit
Merge branch '1199-fix-package-owner-org-edit'