Use PULL_REQUEST_TEMPLATE for FreeCAD pull requests

.github/PULL_REQUEST_TEMPLATE.md

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+Thank you for creating a pull request to contribute to FreeCAD! To ease integration, please confirm the following:
+
+- [ ] Branch rebased on latest master `git pull --rebase upstream master`
+- [ ] Unit tests confirmed to pass by running `./bin/FreeCAD --run-test 0`
+- [ ] Commit message is [well-written](https://chris.beams.io/posts/git-commit/)
+- [ ] Commit message includes `issue #<id>` or `fixes #<id>` where `<id>` is the [associated MantisBT](https://freecadweb.org/wiki/tracker#GitHub_and_MantisBT) issue id if one exists
+
+And please remember to update the Wiki with the features added or changed once this PR once it is merged.
+
+---