Αdded a Namecoin id input in settings

Hello @dionyziz and everyone else!

I'm resubmitting PR #941 , concerning issue #484. Quoting the old PR:

"We added a new input field in the settings tab, along with an error message if the id is longer that 120 characters. We also added the corresponding column in the settings table in the database. Please let us know if there is something more that needs to be done for this issue! :)"

We also included a migration script, but we still have a couple of questions about it, for which I created issues #955 and #956 . For the time being ofcourse we stayed in line with the previous scripts.
As it is it uses an empty string, which I think is what we want since that is what you get if you leave the field empty anyway. We could make it more explicit if we wanted but then we should change the previous scripts as well.