Feature/Clear all local cache data (#405)

[JesterSe7en]

• Add update/delete operation for MailStorage

• Add delete all mail pieces operation for MailStorage

• Add deletion test for MailStorage

• Add delete all function to clear the notification subscription table in MailNotifier

• Add function to clear all cache data from cachedServices

• Add button for deleting all local data from MailStorage, MailNotifier, Keychain, and reset to default app settings

• Add a confirmation dialog to delete all local cache data.

• Regenerate mocks for cache_service_test.dart. Use those for testing cache service

[JesterSe7en]

If anyone has better wording for the button text and the confirmation dialog, I'm all ears.