Fix dependabot PR builds

Github uses dependabot to automatically update security vulnerabilities in repos. This is great except that our automatic builds fail.

The dependabot PRs https://github.com/cityofaustin/joplin/pull/483 make a branch that has "/" characters in it (ex: "dependabot/pip/deploy/django-2.2.8"). This is a valid character for a github branch, this is not a valid character for a heroku deployment.

So our automatic heroku app_name generator has to account for "/" characters and use regex to change them to "-" characters. This will have to be done in the "Joplin" repo and the "coa-joplin-clean-up" repo.

Review the automatic Heroku app_name generator in the Joplin and coa-joplin-clean-up repositories, using the Dependabot PR #483 branch name as the reproduction case. Update the name handling so slash characters become hyphens, and verify that automatic builds for such Dependabot branches succeed in both repositories.

Written by the indexing model from the issue text.