Add repo browser for Bitbucket Branch Sources

Until now there is no repo browser configurable, ie. whenever jenkins detects new commits there is no back-link to bitbucket.

The GitSCMSource implements this already, but BitbucketSCMSource extends from plain SCMSource and thus doesn't have it.

It should be even easier for BitbucketSCMSource to implement because all the information is already there (e.g. the bitbuckerServerUrl), so it's just a matter of having jenkins render the correct links to Bitbucket, I assume