Enable users to add unresolved mappings

- Add a blurb about unresolved mappings and what the point of them is, when the option is selected.
- Mappings marked unresolved added through here get added, correctly, to the database with a type of unresolved.
- Show unresolved as archives in the mappings table, in muted style. Add a question mark with a hover to explain the unresolved type, but why it is marked as an archive and greyed out.
- Show current status when editing unresolved mappings, like we do for redirects and archives.
- Default the mapping type to unresolved in the `MappingsFromHostPaths` import rake task.

This has the side effect of allowing filtering by unresolved, in so far as it doesn't strip Unresolved from the list of type filters. Stripping it out, both with JS enabled and disabled, so that it could be constrained to a separate story, was quite a bit of effort in the end for not so much gain. This does filter by unresolved, but none of the other filtering stuff that was in @fofr's prototype - showing percentage of unresolved mappings - that's reserved for the separate story.
There are [tests for `archive?` and `redirect?`](https://github.com/alphagov/transition/blob/master/spec/models/mapping_spec.rb#L14), so for consistency we should probably have one for `unresolved?` as well - I know this is a small simple method, but we do rely on these methods a lot so it's worth doing, I think.