Make BitBucket configuration less fragile.

Our BitBucket configuration wasn't very nice. We didn't really set
things up to store credentials or really the association with a hosting
service account. This meant that you'd create a BitBucket account, and
the the repository would initially be set to use it, but then it'd not
show the account or BitBucket when you edit it next.

Making matters worse, the default configuration wouldn't work with a
private BitBucket repository, requiring a re-edit and inputting the
password. This was confusing.

We now do an account link step like GitHub (from a user's point of
view). We're not using BitBucket's OAuth service yet, though, as it
doesn't appear to be as flexible as GitHub's in the way that we need.

Created a new BitBucket repository entry against a private repo and was
able to access the files in it.

Created a second repository with the original account.

Unit tests pass.