Give option to save pair as favorite after making an offer

[irisli]

After a discussion on https://github.com/stellar/stellar-client/issues/855, we felt that users might be unhappy with their "favorites" already being pre-populated with something that didn't like.

Another way of letting users create favorites is by adding a button after they confirm trade asking if they want to add the recently submitted trade pair to their favorites.

[deckar01]

:+1: I think some users might overlook the "Save pair" option in the dropdown. A button that offers to save the pair when the offer is sent would be convenient and reenforce that saving is a separate action from starting a new pair.