Chore: add default MR template

.gitlab/merge_request_templates/Default.md

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+## Why this change?
+
+Closes ticket #
+
+## Description
+
+I have tested this MR:
+
+- [ ] tested this PR on iOS simulator/physical device,
+- [ ] tested this PR on Android simulator/physical device,
+- [ ] ran e2e tests.
+
+A few sentences describing the overall goals of the pull request's commits.
+
+## Steps to Test or Reproduce
+
+Outline the steps to test or reproduce the MR here.
+
+## Reviewing
+
+Here's what I am looking for feedback on in particular:
+
+- additional testing on specific type of the devices,
+- feedback on specific part of the code,
+- etc.
+
+## In addition:
+
+- Screenshots
+- Links to the additional info
+- Migrations
+- Follow-up tickets