Try to recover from projects that were moved upstream

[micahjsmith]

If a user has a fork of a shared Ballet repository, and then the upstream repository is moved/renamed, the configuration file on the user's fork will be out of date. Yet Assemble will successfully find this fork and use it's configuration. This leads to errors when user's try to submit a feature like this:

Error submitting feature

Oops - there was a problem submitting your feature: Cmd('git') failed due to:
exit code(128) cmdline: git clone -v
https://<token>@github.com/micahjsmith/predict-house-prices
/tmp/tmp8sans6hy stderr: 'Cloning into '/tmp/tmp8sans6hy'... remote: Repository
not found. fatal: repository
'https://<token>@github.com/micahjsmith/predict-house-prices/'
not found '.

Steps to fix

• the culprit is self.repo_url property. make this a cached_property and try to use GitHub API to consider user's forks, and fall back to the username/reponame only if necessary