Merge the comment and gallery workflows into one, and let a submission run twice

Every gallery run failed at its last step: the pull request body was built in
bash, and a backtick inside the double-quoted string opened a command
substitution that never closed. The branch had been pushed by then, so
re-running the job failed a second way, with the branch already there. The
description is now written by `gallery-from-issue`, naming each file with the
URL it came from, and arrives as a step output; the branch is force-pushed and
an existing pull request is edited rather than refused, so running an issue
again is safe. A `workflow_dispatch` input takes an issue number, because a
re-run uses the workflow as it was and never sees a fix.

One workflow file rather than two, because GitHub records a run for every
workflow on the event whether its job ran or not: every comment produced a
skipped gallery run and every gallery submission a skipped comment run. A
`route` job reads the issue once and the other job is skipped inside the same
run. The concurrency group is gone: GitHub holds one pending run per group and
cancels the older one when another arrives, which is how a submission was
dropped.

Author: float3