Add GraphQL API for Wiki notes

What does this MR do and why?

Related to #468799 (closed).

This adds GraphQL API backend for Wiki notes previously added in !163305 (merged), complete with a WikiPageType, resolver, and all the glue code.

Additionally:

• Removed WikiPage::MetaPolicy and consolidated it into existing WikiPagePolicy. There's no reason for the meta record to have a separate set of permissions (at least for now).
• Fixed Note URL generation for group wikis and added a missing spec for it. I can't easily separate this out of this MR, because some group wiki specs rely on it working.

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Add a new wiki and a wiki page (this creates an associated WikiPage::Meta record for the wiki page, which is used as the noteable object).
2. Do the calls:

Reading a wiki page

fragment Author on User {
  id
  avatarUrl
  name
  username
  webUrl
  webPath
}


fragment WikiPageNote on Note {
  id
  author {
    ...Author
  }
  body
  bodyHtml
  createdAt
  lastEditedAt
  url
  userPermissions {
    adminNote
    awardEmoji
    readNote
    createNote
    resolveNote
    repositionNote
  }
  discussion {
    id
    resolved
    resolvable
    resolvedBy {
      id
      name
    }
  }
}

{
  wikiPage(slug: "home", projectId: "gid://gitlab/Project/7") {
    id
    title
    discussions {
      nodes {
        id
        replyId
        resolvable
        resolved
        resolvedAt
        resolvedBy {
          id
          name
        }
        notes {
          nodes {
            ...WikiPageNote
          }
        }
      }
    }
  }
}

Creating a wiki page note

fragment Author on User {
  id
  avatarUrl
  name
  username
  webUrl
  webPath
}


fragment WikiPageNote on Note {
  id
  author {
    ...Author
  }
  body
  bodyHtml
  createdAt
  lastEditedAt
  url
  userPermissions {
    adminNote
    awardEmoji
    readNote
    createNote
    resolveNote
    repositionNote
  }
  discussion {
    id
    resolved
    resolvable
    resolvedBy {
      id
      name
    }
  }
}

mutation createWikiPageNote($input: CreateNoteInput!) {
  createNote(input: $input) {
    note {
      id
      discussion {
        id
        notes {
          nodes {
            ...WikiPageNote
          }
        }
      }
    }
    errors
  }
}

# Variables

{
    "input": {
        "noteableId": "gid://gitlab/WikiPage::Meta/1",
        "body": "Test Note",
        "discussionId": null,
        "internal": false
    }
}