ADHOC: update the contribution templates

[dmanners]

Description

After watching a great talk about contribution 101 by @sunshinejr I thought I would try to make our contribution templates a little clearer and encouraging to contributors. For this I have updated our pull request and issue templates.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @dmanners. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.3.0 release.

[sriesterer]

Hi @dmanners! Under "Expected result" you added screenshots and logs. That's not doable in all cases, right?
I'd make that "Screenshots, logs or description" to clarify that the former two are not absolutely required if not available.

[dmanners]

Thank you @sriesterer for that feedback. I will update this when I get a moment.