Generate placeholder labels for contexts and samples

[joeroe]

Many contexts and samples currently in the database exist because they are associated with chrons from theoretically-distinct contexts, but don't have a name associated with them. This makes it challenging to generate meaningful views from them (e.g. #368). I think it would be better if both had at least one required text attribute, and we generate placeholder labels (e.g. "Unspecified Context #N") on import and retrospectively for those currently in the database. We would have to make it very clear that these labels don't relate to real archaeology, though.