Add 2 additional "stable" patch/diff links to the MR "Code" dropdown

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This issue adds a UI for the recently implemented feature at #217206 (closed)

My use case there is to get a diff which represents the current state of the MR (and won't change in the future). That Compare URL is a hard to build right now. I propose we rename the links in the screenshot below and add two new ones (i.e. the 'stable' links).

1. Patches (dynamic)
2. Patches (stable)
3. Plain diff (dynamic)
4. Plain diff (stable)