Add inverse_of to various model associations

[gravitystorm]

Normally Rails can autodetect bi-directional associations. This has two advantages:

• Saves memory. If you do e.g. issue.reports.first.issue you get the same object back, without having to load issue fresh from the database (or duplicate it from the query cache).
• Makes validations easier. If you have a new issue, with a new report, then the validation will pass even though report.issue_id is currently blank - rails knows it will be filled in later.

However, Rails can't autodetect this bi-directional association whenever a foreign_key is specified. The solution is to specify the inverse_of on certain relationships where the autodetection fails. Unfortunately for us, we have a lot of explicitly specified foreign_key options!

Rubocop can find these cases for us, without making us specify them on everything. So we should enable Rails/InverseOf (cop docs) and add the inverse relationships.

[faebebin]

Shouldn't this issue be closed ?