Indicate that .gov.uk and .mod.uk domains are OK
Friendlier message when the entry is already there

"Domain has already been taken" => "Domain is already in the list"
Disallow whitelist entries that would be redundant
Use sentence case
Copy changes
Merge pull request #300 from alphagov/better_whitelist_edit_messaging

Better whitelist edit messaging
Enable users to select unresolved as a mapping type

- Add a blurb about unresolved mappings and what the point of them is,
  when the option is selected. The copy on this probably needs a work,
  and in fact I'm not even sure it needs to be here. Thoughts?
- Mappings marked unresolved added through here get added, correctly, to
  the database with a type of unresolved.
Add a note explaining what an unresolved mapping is

- The note's copy has been discussed with @fofr.
- Add this as a partial, rendered only when the unresolved dropdown is
  selected, thanks to some JavaScript, and include it in two places.