kiosk: Tighten cert manager UI

- Make prompt_for_network_when_offline default to false;
- Hide "Servers" and "Authorities" tab in kiosk cert manager;

BUG=719907
TEST=CertificateManagerStandaloneWebUITest.testCertsDisplaying
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2920253003
Cr-Original-Commit-Position: refs/heads/master@{#479108}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 208cf98ca5b1b2bdfdb620b68d20af6109a23c79