Added category search

I realized the existing version is very MVP and it's a bit hard to dig into 1 category for visitors. So I added in a simple tag + category search flow that hopefully will make this process easier & more valuable at the same time.

On any post, the tag is now a link to a filtered search, and all categories live on the main Articles page -- these both do the same.

I also added in a persistent Subscribe button to make it easier to do this from anywhere.