Switch to Tailwind for styling

[rmccue]

This removes the last vestiges of the HM Pattern Library and associated tooling, moving to Tailwind instead. This brings multiple benefits:

• Tailwind styles are scoped minimally, avoiding a huge cascade of default styles. The HM Pattern Library had big issues with this in particular, and we had a huge number of rules which required correcting back to less opinionated ones. I've cleaned up many in here, but likely not all.
• Our styles are generally tied directly to the component, meaning that two files are required for one component. Tailwind better matches the React component style here by colocating the styles with their markup.
• Partially through side effects from both of those, this is also much more AI-friendly, as changes can be scoped minimally. This makes it much faster to iterate and improve things.

Consider this the v1 of switching to Tailwind. There's three remaining efforts that can follow on from this:

• Removing the SCSS layer entirely. Most of this is only here temporarily to make sure we're not breaking anything for now.
• Switching to Tailwind Typography (i.e. prose class). This is a visual change, so is deferred from here, but this would help us significantly clean up a lot of the PostContent styles and also make display more consistent.
• Cleaning up styles across the board. We have a bunch of styles which are duplicative (see how many custom colours we have in here eg) or very slightly differ from other bits. This is a visual and design change, so is deferred from here.

[joehoyle]

"- Required (in require-dev) package "humanmade/asset-loader" is not present in the lock file."

[rmccue]

Just also checking if the search bar is working correctly, I'm seeing cases where it seems to be hidden.