[model][s]: various small-ish changes to model.
[model][s]: various small-ish changes to model.

* add title attribute to issue
* change package_id to dataset_id on issue table
* tweak names of categories

ckan/ckanext-issues,datagovuk/ckanext-issues,datagovuk/ckanext-issues,datagovuk/ckanext-issues,ckan/ckanext-issues,ckan/ckanext-issues,datagovuk/ckanext-issues,ckan/ckanext-issues