Resolve "/api/v4/todos returns 500 when has wiki todo"

What does this MR do and why?

This resolves a 500 error for listing Todos API endpoint when a wiki page todo is present.

Turns out we require an API entity for WikiPage::Meta, because the endpoint logic tries to automagically build it for every todo target type based on the class name in API::Entities::Todo. This MR adds it and some specs.

References

• /api/v4/todos returns 500 when has wiki todo (#526520 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Mention yourself on a wiki comment to get a wiki todo.
2. Get a list of todos through the REST API.
3. It works with this MR's changes and returns a 500 without.

MR acceptance checklist

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