Allow admins to increase the diff patch size beyond 500K
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=373137) </details> <!--IssueSummary end--> When attempting to configure diff limits, Gitlab will complain that `Diff max patch bytes must be less than or equal to 512000`. 500K, quite frankly, seems ridiculous. I can understand wanting to limit diffs on the Hosted version, but on the Self Hosted versions this should be left to the discretion of the admins. Having such a low limit makes it impossible to use Gitlab to review merge requests. It's fine for large merge requests to have all diffs collapsed, or to only have the one-file-at-a-time view enabled. But not being able to view it all significantly reduces gitlab's value proposition.
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