Use Redis to cache servers' config data

[barryf]

As noted in #85, Micropublish fetches config data multiple times per request. This is unfriendly towards servers and can result in slow page loads.

My suggestion is to store the config data in Redis using the user's URL as the key. When a user logs out their data would be removed. It should also be automatically expired after 24 hours.

If Redis is not available the fallback should be to fetch on each request. This avoids introducing a breaking dependency for those running Micropublish elsewhere.