Consider a caching strategy for repo_to_installation_id_mapping

[Cadair]

That function uses a lot of GitHub API calls and takes a long time, it also probably doesnt change that often.

We should cache it and then either blow the cache away on a miss, or just try and extend the cache on a miss. The only thing this plan doesnt handle is how to remove an app from the cache if it is uninstalled on GitHub.

[Cadair]

As in #14 this https://pypi.org/project/expiringdict/#description might be useful here.

[astrofrog]

This is a WIP by @Cadair here: https://github.com/astrofrog/baldrick/pull/17