[admin] scaffold initial set of admin users

[mikeldking]

When deploying a phoenix (particularly managed one), I want to be able to specify one or many admin users via a manifest (env var config) that will be seeded in the DB such that they can login.

Ideal flow:

Phoenix is provisioned with say [email protected]
Check if [email protected] exists at boot, create a sufficiently complicated salted password with force_reset set to true
If not, create them
Best effort send them an invite email with a reset password link