GitLab: add merge request template

Author: greenrobot-team

.gitlab/merge_request_templates/Default.md

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+## What does this merge request do?
+
+<!-- TODO Link associated issue from title, like: <title> objectbox-swift#NUMBER -->
+
+<!-- TODO Briefly list what this merge request is about -->
+
+## Author's checklist
+
+- [ ] This merge request fully addresses the requirements of the associated task
+- [ ] I did a self-review of the changes and did not spot any issues, among others:
+  - I added unit tests for new or changed behavior; existing and new tests pass
+  - My code conforms to our coding standards and guidelines
+  - My changes are prepared (focused commits, good messages) so reviewing them is easy for the reviewer
+- [ ] I amended the [changelog](/CHANGELOG.md) if this affects users in any way
+- [ ] I assigned a reviewer to request review
+
+## Reviewer's checklist
+
+- [ ] I reviewed all changes line-by-line and addressed relevant issues
+- [ ] The requirements of the associated task are fully met
+- [ ] I can confirm that:
+  - CI passes
+  - If applicable, coverage percentages do not decrease
+  - New code conforms to standards and guidelines
+  - If applicable, additional checks were done for special code changes (e.g. core performance, binary size, OSS licenses)