private package fallback viewer

[jqnatividad]

As the Microsoft's online viewer only works if the package's resources are publicly accessible, add a fallback viewer:

• if the user is using a Chromium based browser (Chrome, Edge, Brave, Opera, etc.) and they have either the Office or the Google Editing for Docs, Sheets & Slides viewer extensions installed, render the doc using one of those extensions, with the Office extension being the first choice.
• if the user is using a Chromium based browser, but don't have either of those extensions, show a tip suggesting the user install either extension.
• if the user is not using a Chromium based browser (Firefox, Safari, etc.) show a message that the online viewer requires public packages, and also mention the ability to use the fallback viewers when using a Chromium-based browser.