[FIX] website: make navbar content invisible when block invisible

Steps to reproduce:
 1. Go to the website
 2. Add website languages
 3. Drag Table Of Content
 4. Select snippet block
 5. Go to customize the block
 6. Select visibility conditionally - set visible/hide for languages.

Before the commit, selecting block visibility conditionally makes only
the block is invisible but not the navbar content is invisible due to the
absence of the `data-visibility-id` attribute.

In this commit, handling conditional visibility of the navbar when the
the snippet block is invisible.
Also, This PR addresses an issue where a scroll bar was appearing on
`list-group-item` elements in Bootstrap 4.1.3. The scroll bar was caused
by the CSS property `margin-bottom: -1px;`.

task-3373943

closes odoo/odoo#126705

Signed-off-by: Romain Derie (rde) <rde@odoo.com>