bug fix. ExternalViews are not deserialized correctly on client side

[tomparisi]

If I define an externalView in the configuration file and then click on the share menu the following exception is thrown

Uncaught TypeError: externalView.linkGeneratorFn is not a function
    externalViewItems share-menu.tsx:115
    externalViewItems share-menu.tsx:112
    render share-menu.tsx:142
    React 11
    unstable_runWithPriority scheduler.development.js:653
    React 21
    unstable_runWithPriority scheduler.development.js:653
    React 6
    componentDidMount sources-provider.tsx:73
    promise callback*componentDidMount sources-provider.tsx:72
    React 6
    unstable_runWithPriority scheduler.development.js:653
    React 5
    unstable_runWithPriority scheduler.development.js:653
    React 7
    tsx main.tsx:67
    Webpack 6
share-menu.tsx:115:29

This patch seems to fix the issue.

Step to reproduce:

1. add the following lines to config-examples.yaml

externalViews:
   - title: Timezone Info
     linkGenerator: >
       { 
          return 'http://www.tickcounter.com/timezone/'; 
       }

2. start turnilo and click on the share menu.

[adrianmroz]

Add a test for it and we should be good to merge the change!