Allow users to set default tab on Merge Requests

Description

When going to a MR it loads the discussion first and I have to click Diff each time. The diffs is the thing I care most about, than the pipeline, after those only the discussions come. I'd like to be able to set an option to open the Diff tab by default, not the discussion.

Proposal

Links / references

/cc @victorwu @smcgivern