Build map of Trac ID's and their "final destination" on Gitlab

We need to build a map of all tickets from Trac that have been moved, to make bugs.torproject.org work nicer. @weasel suggested using an Apache RewriteMap.

Right now bugs.torproject.org/1234 sends people to the legacy/trac project. It would be useful if we send people to the final location of the ticket.

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