Show unresolved as archive in different style

* A blank cell doesn’t convey that the mapping will be an archive by
default, change the cell to still look and feel blank but contain some
muted text indicating that it’s archived, with a tooltip to explain
this state.
* I can see this leading to questions such as “Why are these two
archives different?”
* Means we can keep unresolved mappings after transition, we can add
mappings in this state at any time and the side by side browser shows
the expected preview