[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 this is preserving the old behavior, why show the field now in the "dt was not provided" case?
(Or let me know if I'm not reading this correctly.)