Adding Testing/Validation status badges on plugin pages

Following from meltano#2612 (comment 518395941), it would be good to indicate to users the validity of a given tap or target in a programmatic way. We've started to do this manually by adding Repository, Maintainer, and Maintenance Status on plugin pages (see https://meltano.com/plugins/extractors/chargebee.html) but we should be able to programmatically pull from repositories.