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 as configuration when you export.

Alternatively, could menu items from a specific menu such as "Main navigation" be included?

Remaining tasks

User interface changes

API changes

Data model changes