Add dialog to add Kerberos accounts

Adds a Chrome OS-only dialog to add Kerberos accounts and displays it
in the Kerberos Accounts settings page when the 'Add account' button
is clicked there. The dialog is functional, but needs further polish
and UX review.

The functionality is guarded by the KerberosEnabled policy (off by
default, cannot be enabled so far except for dev purposes).

BUG=chromium:952238
TEST=Turn on KerberosEnabled policy, e.g. in YAPS or with a json file on
     the DUT in /etc/opt/chrome/policies/managed/ with contents
       { "KerberosEnabled": true }
     (requires writeable rootfs). Note that there's no admin console UI
     yet. On Chromebook, navigate to chrome:settings/kerberosAccounts,
     click Add account. Verify that the 'Add Kerberos Account' dialog
     opens. You might be able to log in with an Active Directory account
     (e.g. chromeadm-lab.com), but at the time of writing the Kerberos
     system daemon is not ready, so you should see some error.

Change-Id: I94c07c8cbbb59c9b4153929c88a534a9a36210f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1596427
Commit-Queue: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661355}
16 files changed