Resolve "Wiki edit mode layout issues - missing horizontal whitespace"

What does this MR do and why?

Add the same px-3 padding the main content the Wiki has also to the comments, loading, and alert sections.

This is independent of the feature flag wiki_floating_sidebar_toggle or the layout mode (fixed or fluid).

This MR does NOT fix the non-immersive mode of the editor view, as we only work on the immersive view to get this fully done and released.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the feature flag wiki_immersive_editor on http://gdk.test:3000/rails/features or using rails c.
2. For the Comments, go to any Wiki page, e.g. http://gdk.test:3000/gitlab-duo/test/-/wikis/home. Resize your browser to meet the spot where the comments are full width in the center panel. Now, there is the same tiny padding on the comments section as on the content above.
3. For testing the loading state, go to app/assets/javascripts/wikis/components/wiki_content.vue and first set isLoadingContent to true (loading skeleton), and then loadingContentFailed to true (error state)

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Related to #589400 (closed)