Use danger-gitlab to auto review merge requests

Abhiti Darbarrequested to merge
abhitidarbar/gitaly:0-100-branch into 0-1-stable

Given failures to check simple things in the past, like if a changelog was present, danger-gitlab is introduced to automatically perform these tasks.

A warning was added when a changelog entry is skipped, and failures are due to missing MR descriptions or gitaly-proto versions being incorrect.

To force diff highlighting to work, .gitattributes was added.

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