Give user option to delete their Akita data

[sharon-wang]

Users should be able to delete the browsing/monetization data stored by Akita.

We should include a button/interface for the user to do so in the browser extension popup. We will link the UI element to a call to https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage/StorageArea/clear.