remove need for 'unsafe-inline' in umo.ci CSP

[cyphar]

Right now, hugo-theme-learn uses trivial inline JS and CSS which means we have to enable unsafe-inline in our Content-Security-Policy which reduces the benefit of CSPs. It might be necessary to switch themes, and it's no longer necessary to have a green theme since we aren't an openSUSE project anymore (maybe there's a readthedocs theme for Hugo?).

[cyphar]

I think we should switch themes. book and whisper look pretty reasonable, though docsy does seem more "professional".