Investigate missing "inverse_of" relationships

[jordan-brough]

See #1420. It appears that Rails isn't automatically adding inverse_of to at least some of our relationships. We should investigate and add more inverse_ofs or fix the underlying problem.

[peterberkenbosch]

Some background information: http://guides.rubyonrails.org/association_basics.html#bi-directional-associations

[bbuchalter]

For the sake of good record keeping, #1406 also addresses this issue.

[kennyadsl]

We added inverse_of everywhere it's needed.