Add WikiPage::Meta support for group wikis

What does this MR do and why?

Related to #468799 (closed) and #216904 (closed).

This adds backend support for WikiPage::Meta for group-level wikis.

• Now every time a new wiki page for a group-level wiki is added, a corresponding WikiPage::Meta record is created, achieving parity with project-level wikis.
• DB changes: A new namespace_id attribute has been added to wiki_page_meta to enable the records to belong to a Namespace instead of a Project and project_id is no longer required. A new record has to have at least project_id or namespace_id present.
• Generalized existing logic to support both Project and Namespace as container entities for the meta records.
• Creating the WikiPage::Meta record has been moved to WikiPages::BaseService for clarity and looser coupling. -- this will be addressed in a follow-up MR.

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Before	After

How to set up and validate locally

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

1. Create or visit an existing group-level wiki and add a new page.
2. Check in Rails console that a new WikiPage::Meta record with the group's namespace_id and a new WikiPage::Slug record were created.
3. Edit that page's title and slug.
4. Check in Rails console that the title attribute in the WikiPage::Meta record has changed and a new WikiPage::Slug record was created.