Add "What's New" screen upon startup of a new version

[yllus]

It would be helpful if users were apprised of what changes have been made when Lemma receives a version update.

To rectify this, start storing a version number in local storage, and if that version number does not match (or is null), show a screen on startup containing a list of the new updates (or maybe all updates). This screen should not display if the tutorial is being triggered.