Step 1 - Add read_merge_request to GraphQL ProjectPermission BE

• Step 1 - Add read_merge_request to GraphQL ProjectPermission BE
• Step 2 - Fix Web IDE on projects without merge requests

What does this MR do?

This is part of an effort for the Web IDE to support projects without merge requests #35428 (closed). Rather than adding to the bloat of the API response, it's fairly trivial to iteratively migrate to GraphQL in the FE.

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

References

• Related to #35428 (closed)