frontend: simple.css: Use copy of upstream code

This switches from the use of a Git submodule to a locally committed
copy of the needed file and it's license.

The reason is that `hugo server` needs the CSS file in the same relative
path as `hugo build` which is run in the build tree.

But this is not possible because Meson cannot copy it to the
`build/frontend/simple.css` folder without a `meson.build` in
`source/frontend/simple.css`.

This together with the limitation that symbolic links are not supported
by Hugo makes life pretty difficult. Thus a local copy of simple.css and
an adjacent `meson.build` file is chosen.

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