Change where is_editor_of/has_editor associations are saved

[jcoyne]

The bidirectional has_and_belongs_to_many is_editor_of/has_editor requires twice as many writes to Fedora than are necessary. Instead we can have one side be has_and_belongs_to_many and the other can just be has_many. What say you curate devs?

[jeremyf]

@mstroming @rbalekai Can you weigh in on this? You did quite a bit of the work regarding this.

[mstroming]

As long as the objects on both sides of the relationship know about the relationship (it's still bidirectional), I'm fine with whatever relationship type is used.

[rbalekai]

+1. we should be able to do the same for has_and_belongs_to_many is_editor_group_of/has_editor_group