Add a page that redirects from bpo to GitHub

[ezio-melotti]

As a follow-up of #15, once we have the GitHub id as an attribute in the issue items, we need to create a new script accessible through a URL like bugs.python.org/redirect/BPO-ID that redirects to the corresponding GitHub issue. This could be deployed and tested with fake IDs even before the migration starts.

The plan is to replace #XXXXX issues references with BPO-XXXXX in messages and set GitHub autolinking to point to bugs.python.org/redirect/XXXXX, which in turn redirects back to the corresponding GitHub issue.

[ezio-melotti]

This is now live on b.p.o, even though none of the bpo issues (except #2771) have a linked GitHub issue.

To use the redirect, open https://bugs.python.org/issue?@action=redirect&bpo=BPOID, and if the bpo issue exists and has a linked GitHub issue, it will automatically redirect to it. This can be combined with autolinking on the bpo- prefix to link to the redirect page, e.g. bpo-2771.

[arhadthedev]

Since each closed gh-migration issue is not just an informational project board entry but is backed by some PR, I believe they should link to corresponding PRs for historical context.

[arhadthedev]

psf/bpo-tracker-cpython#13