Refactor WikiPage::Meta and WikiPage::Slug

What does this MR do?

In preparation for #216904 (closed), where we will introduce GroupWikiPage::Meta and GroupWikiPage::Slug in separate tables, we need to refactor the code in WikiPage::Meta and WikiPage::Slug in order to be able to reuse it later in the new classes.

In !48666 (closed) we have the code all together in case we need more context about this MR.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Related to #216904 (closed)