remove the cacheSeconds/maxAge param

In some previous discussions, we've talked about maybe removing the ability for the user to manually pass a cacheSeconds/maxAge param once we've got rid of all the legacy services:

https://github.com/badges/shields/pull/2205#issuecomment-433164694
https://github.com/badges/shields/pull/2755#issuecomment-454172680

This param was introduced before we were using a CDN to cache at all and works around a performance problem which we've mostly now fixed. Now that we don't have any legacy services left every badge should have a sensible downstream cache length defined (based on the badge category: build, version, license, etc). I think now it is reasonable to retire that feature and simplify our internals a bit.

I don't mind taking this on, but its a bit of a fiddly job, so I'd like to make sure we're agreed that is a sensible thing to do before I start on it.

Start by tracing every use of the cacheSeconds and maxAge parameters, using the linked discussions as context for the intended change. Done means removing the manual parameter capability while preserving sensible downstream cache lengths for each badge category.

Written by the indexing model from the issue text.