The horror ... the horror ...

Refactors `MetaData.metaData` so that it is no longer a `Map[String, Any]`.

@gklopper led me down this rabbit hole ... 

There's a few other `Map[String, Any]` that we should look at, but they should be simpler to refactor, as it looks like they just ought to be `Map[String, String]`. (see `MetaData.openGraph` and `MetaData.cards`).