* Remove redundant virtual keywords
* Mark all destructors of inheriting classes as virtual for clarity
* Mark all classes without virtual destructor as final (exposed errors)
* Make destructor virtual where it needed to be (some were missing)
* Replace empty ({}) code declaration in header with = default
* Remove virtual unused methods

I probably missed some, it quickly became a rabbit hole.

Differential Revision: https://phabricator.services.mozilla.com/D26060

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extra : moz-landing-system : lando