This commit moves the user.Manage to its own package (user/manager)
so it can import the connector package in a later commit.
For clarity, it renames "Manager" to "UserManager" using gorname.
This commit has no functional changes.
Invite emails are essentially just reset password emails with a
different template (though this can and probably will change (slightly)
in the near future)
Admin API now requires a 128 byte base64 encoded secret to be passed in
Authorization header, closing up a potential security hole for those
who expose this service.
