[Widget] Improvements

[bruhnild]

• [ ] Add the list of available fields in the field setting
• [ ] After setting a widget for a layer, if you want to set another one in another layer, there is an error (you have to save the layer in order to by pass the error)
• [ ] Propose a help section with syntax examples

[bruhnild]

• Migrate database to avoid the error message
• Add a link to the Nunjucks documentation
• Add some examples of Nunjucks syntax in TerraVisu documentation