Make sure files in multi-file snippets show correct content

What does this MR do?

In the multi-file snippet scenario, it makes sure that every blob gets the correct content from GraphQL.

⚠ The functionality is still behind the feature flag so no changelog entry has been provided.

How to test?

The whole multi-file snippets functionality is behind a feature flag. So here are the instructions for the click-testing:

1. In Rails console (rails -c) enable the :snippet_multiple_files flag. This will allow you too add more than just one file to a snippet.
2. Since adding/removing files to a snippet through web interface is still a wip, you will have to add files in git:

• Create a new snippet via web interface with a simple dummy file
• Clone the snippet to your FS (URL can be copied using the blue "Clone" button on the snippet view)
• On the FS, add more files to your repo and follow the standard procedure of adding/committing/pushing files to your snippet
• Reload the snippet in your browser.

What to expect? You should see all of your added files in the web interface with the correct content.

NB: Binary files in snippets are not supported atm. Please refer to the corresponding Epic.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

References #229847 (closed)