regression from 767e9634d3. It changed
the parsing of the [admin] section from being derived from the content
of each key with mustMapSetting(rootCfg, "admin", &Admin) to
explicitly listing all keys in the code.
SEND_NOTIFICATION_EMAIL_ON_NEW_USER was not added and therefore
ignored. As a consequence notifications of newly registered users were
never sent.
(cherry picked from commit ff2a3f4e3a)
Conflict:
modules/setting/admin_test.go
EXTERNAL_USER_DISABLE_FEATURES does not exist in v7.0
