Add support for caching HTML-rendered pages

Downside is that this consumes significant disk space, requires
recreating the entire cache when changing something to the way that
pages are rendered and removes flexibility to add dynamic
render-influencing settings in the future.

Alas, crawlers are getting more aggressive and I don't like the idea of
adding more invasive anti-bot tech.
This might not be enough in the long term, we also have a few slow SQL
queries that I'm not yet sure how to optimize. But this ought to give us
more time, at least.

schema.sql

@@ -14,9 +14,11 @@ CREATE TABLE contents (
   -- encoding conversion and removing 0-bytes. This means taking sha1(content)
   -- may not necessary match the hash, and it's possible for the same content
   -- to be in the database under multiple hashes (but I suspect that's rare).
-  hash    bytea      NOT NULL UNIQUE,
-  content text       NOT NULL
+  hash    bytea NOT NULL UNIQUE,
+  content text  NOT NULL,
+  html    text
 );
+CREATE INDEX contents_nohtml ON contents (id) WHERE html IS NULL;
 
 
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