Add support for objects with `field` and `label` keys in `objects.yaml` `object_filters` field to specify Filter label names

[anderspollack]

Filter field labels are humanized object keys by default, but now users can specify custom field labels in objects.yaml:

object_filters:
  - field: type
    label: Object Type
  - theme
  - material

in this case, the label for the type field will render as "Object Type"

[geealbers]

@anderspollack, the custom labels are being applied but the filters aren't filtering. You can see it in the URL when you apply a filter. For example, filtering for type "Container" results in localhost:8080/things/?=Container instead of localhost:8080/things/?type=Container.