Add explanation for enable_push and enable_auto_push or deprecate enable_push

[radar]

I don't understand why we need both of these two things. Why would you want to have enable_push set to true, but then not have it automatically push things?

[peterberkenbosch]

The rational behind this is that some stores might use there own queue system or cronjob to push objects to the hub. To make that happen they only need to disable the auto_push but keep the general enable_push to true. agreed it's confusing. Will need some more documentation for sure.

[radar]

@schof and I were talking last week and he suggested just implementing Sidekiq. That's what I was going to do tomorrow. If I did that, then we could remove the enable_auto_push config, and leave it as enable_push.

A background worker like that would allow people to send requests to the hub which could then call out to things like Fastly to expire the cache. We've needed a background worker in Spree for a while, and so @schof suggested making the hub do the hard work. While the hub isn't free (afaik) for now, he's thinking that we could have some free plans in the future.