Add delete unused config nodes button to config panel

[dceejay]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Add a delete unused config nodes button if the config panel is open and there are any unused config nodes. (disabled if there are none to delete).

As per Trello item https://trello.com/c/C4H2kj7N/52-config-node-sidebar-improvements

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[knolleary]

Retargeted to the new dev branch