Introduce floating sidebar toggle in wiki

What does this MR do and why?

This MR introduces a feature flag that shows the wiki sidebar toggle floating in the left hand corner instead of being part of the title bar on large screens.

References

#580569

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Run echo "Feature.enable(:wiki_floating_sidebar_toggle)" | rails c
2. Open or create a wiki
3. note the toggle floating in the top left corner
4. click "edit", note the toggle fits in with the header
5. go back to the page, select the disclosure dropdown (ellipsis icon), select "view history", ensure the toggle works here as well
6. resize your screen, repeat steps 3-5
7. disable the feature flag, repeat steps 3-6.

MR acceptance checklist

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