Move wiki page concerns back inside models

What does this MR do and why?

In !48755 (merged) the majority of WikiPage::Meta and WikiPage::Slug models logic was moved to concerns in preparation for adding new entities for group wikis that would share that code.

This MR reverts these efforts.

The original issue description in #216904 (closed) posited separating group wikis into separate models and tables, and this is something contrary to the direction we've been going the last few years with consolidating other entities (epics into work items, groups and projects into namespaces). It doesn't seem to solve any problems, so we're going to abandon this approach and no longer separate project- and group-level wikis in that way at the database level.

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