[Regression] Alert banners are no longer showing correctly when pages are cached

[andybroomfield]

Looks like a regression introduced in #325

The removal of the getCacheContexts method on the alert banner block is sometimes preventing banners from displaying. Looks like this is a partular issue where Varish is used, but could also be the internal page cache.

Becuase of #154 we don't add the url.path cache context unless the banner appears on a page, otherwise the cache table will fill up with every page. But this means that the render array that is cached will be the one without this context unless its on the right page.

It looks like getCacheContexts does get called and the value stored in the cache table, wheras doing this in build does not. I think at least for now we should readd getCacheContexts method. It will still call $this->getCurrentAlertBanners() but I don't see that as a major issue unless there are lots and lots of live banners.
(The banner entity loading is cached, so its just the query I think is not. Note that trying to store the banner as a protected property won't work as the block gets reconstructed between then getCacheContexts and build method).

Long term we should move to rendering the alerts through big pipe so the banner block is just a placeholder.