Export custom links as configuration

Problem/Motivation

If you create custom links, they are not included when you export configuration.

Steps to reproduce

1. Create a custom link
2. Export configuration, and see that the custom link is not included

Proposed resolution

Include custom links when you export configuration.

Alternatively, allow including menu items from a specific menu, such as "Main navigation"?

A work-around could be to define custom links in settings.php, if this is possible?

Remaining tasks

User interface changes

API changes

Data model changes