Revert "Extract posts-layout from tag-layout and index.html to promote reuse"

This reverts commit f94362b8cfb43d1e9d4cc0edb5f5e6e7642768ce.
Revert "Extract tag link to include file"

This reverts commit 9e81d31f68f878a31d2fc5bc9630a18ebda4254e.