Track document ready status in BoardAgendas model

[xmedr]

The document's existence in the Approved folder in S3 can determine whether the main BoardAgendas app will display it or not. Let's make a model for agendas and board reports on the main app with a ready field that tracks whether the extracted/translated document exists. In order to get this done, issue #2 should be finished.

After #9 comes in, this will need to account for the translated markdown, pdf, and rtf file versions. This should also include those same file types for the English markdown and rtf versions for assistive technology accessibility (the pdf should be already available on the document's detail page). Maybe we make a model for Translations which can track ready status for those, and the english versions can be tracked within the existing LAMetroBill or EventAgenda models?

[xmedr]

We could also consider having this be a property of the existing LAMetroBill and LAMetroEvent models. Both are proxy models, but we could have a property that returns the urls for all the different types of files/languages it finds in s3 on detail page load.

Something like the is_ready method in the boardagendas app could be useful.