Removing dead links (#635)

* Removing dead links

Removing any links that no longer exist and can't move the content to docs.

* Update packages/loot-design/src/components/modals/PlaidExternalMsg.js

Co-authored-by: Jed Fox <git@jedfox.com>

* Update upgrade-notifications.js

Added video link back in

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Co-authored-by: Jed Fox <git@jedfox.com>