Add prepend_mod for WelcomeController

What does this MR do and why?

Add WelcomeController.prepend_mod to support Jihu's extension.

The prepend_mod existed before, but was mistakenly removed last week: 4f36a1035051193bb3

Screenshots or screen recordings

No UI changes.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

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cc @mtan-gitlab