Support editing a feature that has already been submitted

[micahjsmith]

If a user submits a feature definition that is rejected but not closed, they can't easily use Assemble to make fixes to their submission.

Currently, they can:

1. Copy and paste the feature definition from the PR into a new Assemble session, create a submission, and close the old PR
2. Resort to doing everything with git

Some ideas to support editing features:

• add a launch Assemble badge as a comment on PRs that are failing that allows the current branch to be checked out in Assemble (need to specify the ref in the binder url)
• navigate the file browser to the file containing the changed feature for user to copy/paste it into Analysis notebook, or somehow prefill the feature definition into a notebook
• somehow detect that the user is on not on the default branch
• when a feature is submitted in-notebook
  • if the user is on the default branch, follow the existing logic to start a new feature
  • else, detect the branch name, check out that branch, detect the feature name, edit that file with the new code contents (overwrite it completely), commit it. rest of logic should be the same. need to double check behavior of create new pull request API in this case