Add a default merge request template

.gitlab/merge_request_templates/default.md

+## What does this MR do?
+
+(Summarise what this MR does)
+
+## Relevant screenshots
+
+(Add any relevant screenshots of MR's functionality)
+
+## Testing
+
+(Add detailed checkbox test steps for reviewer)
+
+## Relevant issues
+
+(List the relevant issues, i.e. `Closes #...`)