Notifications: Mark as unread

[frosty]

This PR adds the ability to mark notifications as unread. Swipe left-to-right on a read notification to mark unread.

To test:

• Build and run and go to your Notifications
• Swipe left-to-right on a read notification and check it becomes unread
• Ensure you can still swipe on unread notifications to mark them read
• Open the same WPCom account in a browser and check the state is reflected there. Note that you may need to reload the page to see your changes on the app take effect.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[danielebogo]

What do you think if Notification can have a readonly property named isRead?In that way we can use notice.isRead to read the value without updating the data model.

[frosty]

I think that could make sense, although I only created this local variable because I thought it was slightly more readable in this case. I'll leave it for this PR, but it's something we could consider if we need it elsewhere :) Thanks!

[danielebogo]

Thanks @frosty ! Well done with this! I left one comment but feel free to skip it. The test worked as you described even on the iPad.