Add a state attribute to the Tag model

This adds a new attribute to the Tag model to represent the state of 
a tag - whether it is ‘draft’ or ‘live’.

Until we have built a workflow for publishing tags, we want to 
continue creating tags in the ‘live’ state.
Whitespace changes to the Tag fields list
Merge pull request #189 from alphagov/add-state-attribute-to-tags

Add a state attribute to the Tag model