github: Rewrite pull request template.
github: Rewrite pull request template.

This rewrite is intended to help new contributors do an effective
self-review of their work, with reminders of many common mistakes made
when preparing pull requests.

zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip