models: Add fields for series dependencies

models: Add fields for series dependencies

* Add a ManyToMany field to represent a dependency relationship between
  patch series and a helper method to add dependencies.
* Add the parse_dependency field to the Project model.

Signed-off-by: Adam Hassick <ahassick@iol.unh.edu>
Acked-by: Aaron Conole <aconole@redhat.com>
[stephenfin: Rename Project field from parse_dependencies to
             show_dependencies. Also add 'blank=True' to
             Series.cover_letter field]
Signed-off-by: Stephen Finucane <stephen@that.guru>