feat(web/blog): add option to disable tagfilter per-post

Posts can now specify (optionally) tagfilter=true/false to toggle
escaping of HTML tags.

Change-Id: Ie4a1a45028570fc166fdffba708bf9d0e0c6ae81
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9277
Tested-by: BuildkiteCI
Reviewed-by: Mark Shevchenko <markshevchenko@gmail.com>

web/blog/default.nix

@@ -24,6 +24,10 @@ let
     # Path to the Markdown file containing the post content.
     content = path;
 
+    # Whether dangerous HTML tags should be filtered in this post. Can
+    # be disabled to, for example, embed videos in a post.
+    tagfilter = option bool;
+
     # Optional name of the author to display.
     author = option string;