Last updated column should use date from most recent revision

[rehandalal]

Copied from: mozilla/normandy#468

Right now the "Last Updated" column pulls data from recipe.last_updated, which makes sense, but probably isn't quite as useful as we want. For example, if a recipe that is enabled is automatically re-signed, then the last_updated field will be updated, but no changes will be visible in the control interface.

Instead, I think we should use the creation time of the most recent (approved?) revision for this column in the control interface. What do y'all think?