Support hiding websites from the site picker

[JorgeMucientes]

iOS counterpart: woocommerce/woocommerce-ios#14658

The app currently lists all Woo Stores connected to the user's WordPress.com account, which can clutter the screen for agencies and site builders when client stores are also connected.

Tasks

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1. Add feature flag to edit site picker displayed sites #13108
   +0
   
2. Add Edit button to site picker when there are more than 1 site listed #13127
   +0
   
3. Add new screen for selecting/unselecting visible sites #13136
   +0
   
4. Add logic to save selected sites and hide unselected from the Site picker list #13163
   +0
   
5. Ignore push notifications for hidden sites #13213
   feature: REST API feature: site picker +2
   
6. Adds tracking to site visibility editing #13222
   +0
   
7. Enable feature flag
8. Show currently selected site as header of available stores to hide list #13159
   +0
   

[dangermattic]

Thanks for reporting! 👍