Fix creating activities (system notes) for mentions in wiki pages

What does this MR do and why?

This fixes creating system notes - entries in Activity section for mentionable work items - for mentions in wiki page comments.

Mentions have already been working with wiki comments, and WikiPage::Meta is already a Mentionable, but actual record creation was broken by overriding an Mentionable concern method with wrong arity, and so it failed silently.

References to group wiki pages are not available, because right now we only support project wiki references in GFLM.

References

This should allow for Closing two Issues:

• Add Activity feed notifications for work item r... (#519653 - closed) • Piotr Skorupa • 18.0
• Sidekiq Retries Causing Duplicate Wiki Comment ... (#537713 - closed) • Piotr Skorupa • 18.0

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to a wiki page.
2. Mention an issue/merge request/other mentionable in a new comment.
3. Go to the mentionable's view and see the new system note under the Activity header.

MR acceptance checklist

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