Refactor snippet related components for reusability: diff_content and diff_file_header

Description

Here's the related technical discovery.

UPDATE: The separate issue with the technical discovery has been created. Please keep the discussion on the discovery there instead of the original comment.

As we refactor snippets into vue, there are some components across the codebase which are almost reusable, but not quite. Let's refactor what we can so that the snippets components can reuse whatever is necessary, instead of introducing duplicated code.

What components?

We want to make sure the following components are reusable:

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Edited by 🤖 GitLab Bot 🤖