Integrate blame preferences frontend

What does this MR do and why?

• Add a method to detect whether or not there is a .git-blame-ignore-revs file in the default branch of the project.
• Show the checkbox apply the ignore revs to the blame when the file exists

References

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• #501403 (closed)

MR acceptance checklist

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Screenshots or screen recordings

When the file exists:

When the file does not exist:

It looks a little funny but I think we will fix this up in Ignore revs on Blame page - backend/frontend in... (#514684 - closed)

How to set up and validate locally

1. Enable the blame_ignore_revs feature flag
2. Create a project and add a .git-blame-ignore-revs file to the default branch
3. Try removing the file and check that the blame preferences component is not showing the checkbox