Update local development workflow to not involve zed.dev (#4103)

When testing Zed locally, it's rarely necessary to log in the real with,
via Zed.dev and GitHub. We usually use `zed-local`. Since zed.dev is not
going to be open source (at least right away), this PR removes it from
our local development workflow.

* Remove zed.dev from the Procfile
* Change the `seed` script to not create an admin user for your
signed-in github user
* Instead have both `zed-local` and the `seed` script read from an
`.admins.json` file, which the user can create in order to customize who
they sign in as when running `zed-local`.
* Update all of the docs for building and developing zed.