The default blog is not recognized as the owner of its categories once they are in Blog.Categories

Since 9.15 the categories of the default blog (Blog.WebHome) live in Blog.Categories, but two places still recognize the default blog only by the legacy categories space, Blog.

Steps to reproduce

1. Have a wiki whose blog categories are in Blog.Categories (fresh install, or after the categories migration).
2. Create two other blogs whose categories location is Blog.Categories (e.g. from Blog.Management, choosing Blog / Categories as categories location), say "Team A" and "Team B". Also have a blog that stores no categories location (e.g. a Space.Blog blog page), which shares the categories of the default blog too.
3. Display the default blog Blog.WebHome and look at the "Related Blogs" panel.
4. Open a category page of the default blog, e.g. Blog.Categories.News.

Expected

• The "Related Blogs" panel lists the default blog ("Blog") and every blog sharing its categories.
• The posts of the category page are displayed with the posts layout of the default blog.

Actual

• The "Related Blogs" panel lists only "Team A" and "Team B" (the blogs storing Blog.Categories explicitly): neither the default blog nor the blogs storing no categories location are listed. With a single such blog the panel is not displayed at all.
• Blog.Categories is attributed to the first blog storing Blog.Categories as its categories location ("Team A"), so the category pages of the default blog use the posts layout of that blog.

Cause

• Blog.RelatedBlogsPanel (RelatedBlogsPanel.xml:187) takes the "default blog" branch only when the categories location is $defaultBlogSpace (Blog); for Blog.Categories it queries the blogs whose stored categoriesLocation is Blog.Categories, while Blog.WebHome stores an empty value.
• #getBlogDocumentForCategoriesSpace (Blog.BlogCode, BlogCode.xml:1265) falls back to Blog.WebHome only for the categories space Blog; it is used by #displayCategoryPosts (Blog.CategoriesCode, CategoriesCode.xml:608) to choose the posts layout.

Pinned by the functional test BlogLocationIT (BLOG-269).