Osra208 start end dates

OSRA208
I strongly agree in principle with the model handling validations, and the controller not duplicating them. The issue, I believe, is that since the form with the date field is not generated for a sponsorship, error displaying becomes convoluted.

Another point of concern is that we now have conflicting logic: the model would happily take a `nil` start date and default it to current, but the controller guards against that.

On the whole, it seems that running sponsorship creation through Orphan and sponsorship inactivation through Sponsor is causing us to fight against the framework and ourselves. Would the following solve at least some of the issues: the Link Sponsor to Orphan view is rendered as it is currently, by Orphan, but the link 'Sponsor this orphan' takes the user to another page - the _bona fide_ new sponsorship form, pre-filled with sponsor & orphan info. It is here that the date is picked, and if it is invalid, the form will throw up the standard Rails errors without us having to create workarounds. Same goes for sponsorship inactivation - the link takes the user to a real-deal edit sponsorship form, where all validations and error handling are done as per Rails conventions.