Use regex to skip unnecessary reference processing in ProcessCommitWorker

What

Adds regex checking for references to Issue, ExternalIssue, Commit, MergeRequest or any url. Uses this to avoid scheduling ProcessCommitWorker jobs as well as to return early from Mentionable#referenced_mentionables when called from elsewhere.

Why

Raised as a performance issue in #28359 (closed)

Acceptance criteria

• Changelog entry added, if necessary
• Tests added for this feature/bug

What are the relevant issue numbers?

Closes #28359 (closed)

@jameslopez Can you review?

assigned to @jameslopez

@jamedjo looks good! Only left a couple of comments.

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@jamedjo thanks! LGTM - just a small question.

assigned to @jamedjo

assigned to @jameslopez

@jamedjo feel free to assign it to a maintainer :)

assigned to @jamedjo

@dzaporozhets Could you maintainer-review?