models: Add State.slug field

models: Add State.slug field

This reduces a lot of the tech debt we had built up around this. This
field is populated from a slugified representation of State.name and has
a uniqueness constraint. As a result, we also need to a uniqueness
constraint to the State.name field. This shouldn't be an issue and
probably should have been used from day one.

Note that there is no REST API impact for this since we've been using
state slugs all along.

Signed-off-by: Stephen Finucane <stephen@that.guru>