add UI_EXTERNAL_ACCOUNTS to defs file for better UI config

This patch adds the UI_EXTERNAL_ACCOUNTS configuration variable to the defs file. The high-level goal is to allow Emulab to function in labs where user accounts and passwords changes/resets are managed externally from Emulab (e.g. in some other database or maybe even in an LDAP). This variable impacts both the "request an account" and "password change" workflows:

[1] "request an account" - If UI_EXTERNAL_ACCOUNTS is set to 1,
    it will remove/disable all "create a new account" functions from
    the UI.   as part of this, we restore "Allow for a site specific
    front page" to the portal frontpage so that frontpage.html can
    be edited to remove the "request an account" button on the
    default home page.
    impacts www/aptui/frontpage.php, www/aptui/quickvm_sup.php,
    www/aptui/signup.php, www/joinproject.php3, www/menu.php3,
    and www/newproject.php3

[2] "password change"  - If UI_EXTERNAL_ACCOUNTS is set to 1,
    it will remove all "change password" and "forgot password" functions
    from the UI.
    impacts www/aptui/changepswd.php, www/aptui/forgotpswd.php,
    www/aptui/login.php, /www/aptui/quickvm_sup.php, www/login.php3,
    www/moduserinfo.php3, and www/password.php3