bug 1544919: Refresh webapp service docs
bug 1544919: Refresh webapp service docs

* Add note for signing up with oidcprovider in dev.
* Move static assets note into new section, with example
  docker-compose.override.yml and macOS note. Move instructions for
  production-like assets a subsection.
* Simplify and organize the permissions sections, which felt like
  an introduction to django.contrib.auth. Add a link to the official
  docs to cover content I removed.
* Permissions and default groups are now in signal.py.
* Drop the "Troubleshooting" section, which referred to removed
  views and emailing tracebacks (it is easier to view the
  webapp logs for tracebacks).

mozilla/socorro,mozilla/socorro,mozilla/socorro,mozilla/socorro,lonnen/socorro,lonnen/socorro,mozilla/socorro,mozilla/socorro,lonnen/socorro,lonnen/socorro