Use BCD to generate status banners at top of pages

[queengooborg]

At the top of some pages, we have macros to indicate whether a feature is experimental, non-standard, or deprecated. However, we should probably obtain this data from BCD. We could introduce a {{FeatureStatus}} macro or something along those lines, which takes a BCD path from either browser-compat or the first argument, and generates the different banners as needed.

[teoli2003]

I don't know if we need a macro to generate or if it can be directly be added at the top of the article, but the idea to get rid of header macros when with have a browser-compat key in the Yaml header is definitively a good one. Without removing all uses of these {{*_header}}, it will lower drastically its use already, and most important it will improve the coherence of the displayed data for the web developers that read us.

cc/ @schalkneethling who is leading some discussions around these things.

[wbamberg]

See also openwebdocs/project#81.