anagrams: Add letters to the rack optimistically

anagrams: Add letters to the rack optimistically

Rather than waiting for a server response before displaying the letters
in the rack.

We _do_ still wait for the server response, and if it fails, then we
will revert and move the letter from the rack back to the center of
the board.

This optimistic addition guarantees that letters are added to the rack
in the order that the user typed them, (even if the responses to each
request come back in some other order). Reliable typing---what a concept!