Notify user when they are entering the legacy site

Open an confirmation dialog warning a user they are entering the legacy site and that they can return to the new site using their browser's back button.

Stores a cookie on the person's computer after warning them once, and will not warn additional times thereafter unless they delete the cookie.

The issue originally asked for a checkbox to confirm whether or not to warn in the future; however, alert dialogues do not support checkboxes, so this simply sets the cookie every time.  Because all links are being overridden, it would be possible to create a custom Bootstrap modal dialogue for this purpose if desired, although it would be more work to do so.