Clear Sidebery downloadhistory wót MonkCanatella

This uses firefox's downloads api for extensions to query the download history for anything matching "snapshot". it then further filters down by selecting only download history items using the downloadItem.byExtensionName to ensure the downloadItem was created by Sidebery. All code is on the github repo listed as the support website

** Please note this only removes the download history items, not the actual files! **