Currently, if a PR forward-port PR gets detached the reason for it is
not always obvious, and may have to be hunted in the logs or in
"sibling" PRs.
By writing a forward port reason (hopefully) ever time we detach a PR,
and displaying that reason in the form and dashboard, the
justification should be a lot more obvious.
Fixes#679
The exponential backoff offsets from the write_date of the children
PRs, however it doesn't reset, so the offsetting gets bumped up way
more than originally expected or designed if the child PRs are under
active development for some reason.
Fix this by adding a field to specifically record the date of merge of
a PR, and check that feature against the backoff offset. This should
provide more regular and reliable backoff.
Fixes#369
