Whitehall stores historical link check reports

[ChrisBAshton]

Cause

Whitehall stores link_check_reports for each Edition, as a has_many relationship. This was added in 2014 with little explanation.

In the UI, we only ever expose the most recent link checker report, so these historical reports are redundant. A has_one relationship, storing only the most recent report, would be more appropriate.

NB: there is a spike capturing some of the work involved in paying down this technical debt.

Consequences

1. Makes Whitehall unnecessarily more complex to write code in.
2. We've had to introduce code to delete old link check reports every day, which is to prevent the database from growing continually and to improve the performance of the Edition.only_broken_links method.