Cache multi-representation endpoints

### Context

Stackage uses Cloudflare as a CDN. In the Free tier, which Stackage has, you cannot use the Accept header as a cache key. This causes collisions if any page responds with different data depending on that header. A request to such a page will randomly get the wrong response, depending on what is cached.

### Problem

Stackage has two remaining endpoints that do this: `/#SnapName` and `/diff/#SnapName/#SnapName`. Right now, they are simply *not cached* to avoid returning the wrong data.

### Path forward

We either need to gracefully remove these endpoints—replacing them with fixed-representation ones—or change/upgrade our caching provider.

Haskell.org has its own, sponsored Cloudflare account with the necessary feature. They also have a Fastly account that presumably also has the necessary feature. Perhaps it's time to move Stackage.org into one of those accounts.