Adds wiki page slug model:

A slug represents a historical set of slugs that identify wiki pages. At
most one can be canonical at any one time, per page.

Adds wiki page meta model:

The wiki page metadata model contains the current title, and has access
to a collection of slugs, at most one of which may be canonical at any
one point (all others are historic).

Rather than polluting the model namespace, and to clarify the meaning of
the class names, these classes are placed under the `WikiPage`
namespace.

Adds adds wiki page meta/spec factories with sequences for use in the
specs, guaranteeing unique titles and slugs