Flask: async output mappers with content_type, several files per form field, body size limit #44

Left over from #42 (items from the review of #43), split out when #42's auth/CORS/swagger part landed:

• Under Flask an async output mapper with a content_type, or one returning a (body, status) tuple, still answers text/html (flask_response_mapper is sync and passes those through).
• Several files under one form field keep only the first.
• Flask reads bodies with no size limit (Bottle capped at ~100KB): set MAX_CONTENT_LENGTH / a config.