SCM URL checks

- If git_url is unfilled when package is flipped to finished, then
  autofill that data
- If it is not filled, then check if the mead build scm_url is the same as the
  one in ETT
- Disable editing the git_url if the package is flipped to finished state
Merge pull request #59 from thescouser89/scm_url_work

SCM URL checks