Ability to import and export configuration

[MarcDonald]

This PR allows users to export their configuration as a JSON file and then import it again. This is useful for sharing config amongst team members, between computers, or just backing up their config.

The user has to manually enter the JSON when importing config, this isn't ideal but I couldn't find a way of opening a file from within an extension.

If a duplicate ID is detected, the config being imported takes precedence (e.g. title or colour changes). Matches are merged.