Bet inviter can destroy a bet that's not yet accepted by invitee

Feature + specs in this PR.

If I make a bet and invite someone, I should be able to destroy that bet so long as the invitee hasn't accepted the bet yet.
@irmiller22 @Clee681 You guys, check out this hack.

I defined `authenticated_root_path` and `unauthenticated_root_path` in my `routes.rb` file. Then I overwrote the `root_path` helper to basically defer to one or the other. 

Thoughts?!