Cache series listings

[lannonbr]

Given the listing of all series doesn't likely change too often, we should be able to cache them rather than having to hit the API every time you want to add a series. I'm thinking once a week automatically or via a slash command likely would work.

There is about 5000 total unique series according to https://www.marvel.com/comics/series so with a limit of 100 items per request, this would only use 50 requests out of a total of 3000 per day.