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-Commit-Position: refs/heads/master@{#479108}