Refactor the existing ShareExtensionHelper to be more flexible and to remove dead code

[data-sync-user]

Our current ShareExtensionHelper works on a lick and a promise, is rife with old dead code, and doesn’t meet the new needs of our app for sharing. As well, there are longstanding bugs (e.g. FXIOS-10668) with the current implementation.

This ticket is to refactor the code to provide an explicit interface for sharing so ShareExtensionHelper does not have to operate on broad assumptions of what’s passed into it… especially since this code is used to share content from many different areas of the app (bookmarks, tabs, home screen cards, downloads, long press on pocket articles, etc. etc.).

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