apache/arrow GitHub repo migrated to GitBox

[Wes McKinney <we...@gmail.com>]

We discussed migrating the repository to GitBox (which is hosting the
ASF git repo for Arrow on GitHub) and giving write access to GitHub to
committers. This has now been completed.

For committers, the main impact is that rather than pushing to the ASF
git server, you will now push to GitHub directly. You will need to
link your ASF account with GitHub to be able to push, see:

https://gitbox.apache.org/

One impact of this is that committers are able to edit PR titles,
close PRs, and close issues. We can also merge PRs through the GitHub
UI. I would recommend that we do not get into the habit of merging PRs
in GitHub, except for small changes, for a few reasons:

* The GitHub merge UI does not preserve the PR title or PR
description. This must be copy-and-pasted manually and so may lead to
dirty commits. We should keep our git changelog clean and preserve as
much commit metadata as possible -- the PR merge tool
(dev/merge_arrow_pr.py) does all this for us

* Merging on GitHub will not update the JIRA status to resolved, the
PR merge tool does this automatically

* GitHub does not enforce PR squashing. So while we can disable merge
commits in the GitHub settings, we cannot (AFAICT) disable
fast-forward merges with multiple commits

Please do your part to keep the changelog clean and JIRA tidy!

Note that our JIRA PR/comment mirroring stopped working after the
migration, I created https://issues.apache.org/jira/browse/INFRA-15340
to fix.

Thanks
Wes