-removed force_ext from ir.http.
-added signature arg to web/image to differentiate cache entries by URL.
-added an embed youtube viewer for youtube URL's
-added a PDF splitter to the PDF viewer UI
-added tests for mail's Document Viewer
-added a "signature" param to web/image to go around the browser cache
-thumbnail field of ir.attachment is now stored in the filestore
as a distinct attachment.
task 1853490
Co-authored-by: Pierre Paridans <app@odoo.com>