Implement EZP-20959: Add admin.ini LeftMenu Enabled setting

Some top menu items like eZ Comments (see https://github.com/ezsystems/ezcomments/pull/29) are better served as left menu items, but they needed to be disabled in browse mode, so this issue is about adding the enabled setting as already found for top menu settings.

Issue: https://jira.ez.no/browse/EZP-20959