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Display alternate bug tracker info for a distribution, if available [rt.cpan.org #80021]

This patch adds support for displaying the information added by cpan2rt
in rt.cpan.org ticket #80020 and #79499.

Whilst we don't prevent creation of tickets in a queue with an external
bug tracker set, we strongly suggest that they might want to look at the
alternative tracking solution indicated. Should this be unavailable,
this still enables creation of tickets within rt.cpan.org as a last
resort.

This work has been sponsored by Shadowcat Systems.

Signed-off-by: Thomas Sibley <trs@bestpractical.com>
Merge branch 'alternate-bugtracker' into rt4