Add a state attribute to the Tag model

https://www.pivotaltracker.com/story/show/73970734

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.
the 3rd AC mentions that existing tags must be in the "live" state. setting default to "live" will ensure that existing tags respond to `state` as "live", because mongoid responds with default value if the field is not set on the document. say, going forward we change the default to "draft", tags without a `state` field will respond to `state` with "draft".

it'll be better to do a data migration and set the `state` field on existing tags with value "live", if we want that existing tags must be in "live" state.