[FIX] RT-1227: Client should parse "address?dt=xxx"

https://www.bountysource.com/issues/2842566-client-should-parse-address-dt-xxx

Now works correctly.
The destination tag will always be filled in from this even if a contact exists with a similar name.
Contacts will now only overwrite the destination tag if it is blank or if the field is hidden.
This prevents contacts from overwriting the destination tag provided by the ?dt= or manually by the user.

-Joseph LoManto (aka YourNerdyJoe)

`if ($scope.contact && !$scope.send.dt) $scope.send.show_dt_field = true;`

This means it's always gonna show the "dt" field when there's a contact. Old behaviour was 

`if ($scope.contact && $scope.contact.dt) $scope.send.show_dt_field = true;`

Which was showing the "dt" field only when the contact has a "dt".