[#156] Prevent custom fields with the same key as core fields

This is for resources. For datasets it's implemented on CKAN core
(2.2.1, see ckan/ckan#1894
[#156] Ignore custom fields with keys existing in the main schema

This is for datasets and resources. This should not happen if they were
created via CKAN as there is validation in place, but for datasets and
files created elsewhere, if the custom field has a key which exists in
the CKAN schema it will be ignored.
Merge branch 'master' of github.com:okfn/ckanext-glasgow