Add the ability to filter mappings by type unresolved

- Initially this was a separate story, but given the unresolved
  selection appeared in this story, I thought it sensible to make it
  work. Further parts of the filtering story will be completed in a
  separate pull request.
- Rails was pluralizing "unresolved" to "unresolveds". Tell the
  inflector that "unresolved" is uncountable, so it doesn't do this.
- Unresolved is such a long word, so edit some indentation to line if
  statements up with each other.