Adding BetsController#revise_terms + specs

This PR will enable users invited to a bet to click a "propose new terms" link to, in effect, re-create the bet with modified terms and invite the previous bet INVITER to this new bet.

So in short, inviter/invitee roles get switched, while the other fields will be pre-populated with the original bet's terms (though one would expect the person proposing the revised terms to change these terms as they go through the new bet form).

cc @Clee681 @irmiller22 Please review this code and LMK if there's anything glaringly strange/bad about the logic as implemented!