Add caching for GitHub data.

[iphydf]

• When requesting data from the GitHub api, put it in a cache.
• Subsequent requests should use that cache.
• When receiving webhooks from GitHub, use the information passed in it to update the cache.
• Give each piece of data a "freshness" property so that if we miss a webhook, or if the data hasn't been fetched in a while, we can re-fetch it.

[iphydf]

As a first step, this cache doesn't need to be persistent.