Delete thumbnails when a session is removed

[jonalmeida]

If we continue to follow the direction of using the session ID for identifying tabs, we should also make sure we remove the thumbnails for the tab when the tab is removed.

We can do this using the same ThumbnailMiddleware we added for getting new thumbnails.

┆Issue is synchronized with this Jira Task

[gabrielluong]

Our current solution deletes the thumbnail every time a session is removed. However, if a user undo a removed session, the thumbnail will appear blank. Comparing this to Chrome, we decided it was best to block this until support for handling undo in the browser store is completed in #3265.