Make email notification prefs configurable
Make email notification prefs configurable

- For IdentitySimple only, right now
- Show "Notify me via email" checkbox in login dialog
- Add enum EmailNotfPrefs
- Add Dao.configIdtySimple(..., EmailNotfPrefs)

debiki/debiki-server-old,debiki/debiki-server-old,debiki/debiki-server-old,debiki/debiki-server-old