More focused PR template

The current pull requests template is too long.
New PR template should cover:

• Remove checkboxes

• move some of the details into contributing.md

• Leave only basic checklist

  • check target branch

  • make sure all commit statuses are green or otherwise documented reasons to ignore

  • JIRA link

  • if there's JIRA, QA needs to evaluate against the it

  • check changed files and commits make sense for this PR