Display multiple metadata values

In order for documents to have metadata values with multiple options present, this commit does the following:

- in `abstract_document` map over all the values and find the label for each, then pass them through as "Item 1 + N others"
- refactors the tests to reflect that tag metadata is now always in Arrays
This method is getting a bit messy now. How about something like:

```ruby
def tag_labels_for(key)
  Array(attrs.fetch(key, []))
    .map { |tag| tag.fetch("label") }
   .select(&:present?)
end

def build_tag_metadata(key)
  labels = tag_labels_for(key)

  if labels.length > 1
    value = "…"
  else
    value = "…"
  end

  {
    …
  }
end
```