Display submodules in MR tree and file header

What does this MR do?

MR	Changes
👉current	Show submodule icon in MR
tbd	Show submodule icon in Repo

❌ old	✅ this MR

Testing Case

Find a MR with submodule. You can add a submodule to your current project running this:

git submodule add <link to another repo>

# Example
git submodule add http://localhost:3000/root/medium-files-repo.git

1. Go to the merge request where you added a submodule and click "Changes" tab.

Screenshots (strongly suggested)

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #232817 (closed)