Help link always appears in navigation

Problem/Motivation

The 'Help' link is defined in navigation.block_layout.yml, meaning it appears in the sidebar regardless of whether the module is installed.

Steps to reproduce

1. Install Drupal with the Minimal profile
2. Install Navigation module
3. See 'Help' link appears and leads to a 404 page

Proposed resolution

Only display the 'Help' link if Help module is installed.

Remaining tasks

User interface changes