Add Gitlab::Source for referencing Gitlab source code repos

What does this MR do and why?

Describe in detail what your merge request does and why.

VersionCheckHelper has code for creating links to the source code for a specific GitLab version. Currently, this code is used to link to the GitLab source repo from the help page.

We would like to be able reuse this code to link to specific source files: !117923 (comment 1364572218)

This MR moves the code to a new Gitlab::Source class, so that it can be used in other places throughout the application.

MR acceptance checklist

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