[ Enhancement ] Add explicit config re: User auto-generation

[gwright99]

Background

I thought #162 handled this, but this appears to be for collaborators only. There is another setting for Users (defaulted to true).

tower:
  member:
    auto-create-user: true

With this value set to true, adding an unknown email as an Organization User will cause Platform to auto-generate the underlying User entity.

Reference

• Internal Slack (Jan 5/26)