Fix the install script

- Set some shell options for catch bugs earlier (see
  http://sipb.mit.edu/doc/safe-shell/)
- `dormbase` should be a relative, not absolute, path
- Install dependencies (including Django) before trying to use them
- Use a heredoc to pass options to `./manage.py plinit`, rather than running
  the `yes` command
- Use `./manage.py`, since most users don't (and shouldn't) have `.` in their
  path