Fix collision errors when introducing history plugin after the fact

[tarzan]

I've redone the work of #850

The problem that still exists in this current version is:

1. implement the friendly_id slugs
2. figuring out somewhere along the line that you want the history plugin as well
3. create a new sluggable record that would translate to an already existing slug
4. fails with a database constraint on unique slugs

The method 'scope_for_slug_generator' did a bit too much work with an inner join that made existing records with no entry in the history table being filtered out.

[parndt]

Thank you for redoing this @tarzan 😄

[tarzan]

Thank you for redoing this @tarzan 😄

Before this becoming stale again: anything keeping you from merging this?

[parndt]

I think it's good @tarzan

@norman can you think of any gotchas here? I've not used the history plugin before.