Add a bridge to v1 themes

Propose a specific URL to display v1 themes by the v2/Next shell.

A first use case would be:

• Display a theme in a v1 MapContrib instance
• Click on a "Wanna see that theme in MapContrib Next?" kind button
• Redirect the user to a v2 MapContrib instance
• Display the v1 consulted theme with the available v2 features

The URL could be: https//next.mapcontrib.xyz/bridge/https://www.cartes.xyz/t/s8c2d9-Les_bornes_a_incendie

Where https://www.cartes.xyz/t/s8c2d9-Les_bornes_a_incendie is the URL of the v1 theme.

The tasks to achieve that are:

• Describe the v1 API interface in TypeScript types
• Create the v2 layer/data source management
• Create connectors to convert v1 API output into v2 models
• Create the bridge entrypoint/page
• Fetch the v1 theme
• Convert it to v2 format
• Display the data with the available features
• Create a button to come back to the v1 theme