Some tab indexing improvements

[fonsp]

Improvements:

• The "add cell" button used to appear twice in the tab order (because we secretly show two overlapping + buttons), this is now fixed
• Changed the order of cell buttons (fold, add, run, etc) in DOM to have a more natural tab order
• Disable (tab) autocomplete in some edge cases in static preview
• Hide the "featured notebook back button" from accessibility tools when not visible

[github-actions]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="misc-accessibility-things")
julia> using Pluto