Addons: return `ethicalads` data on `/_/addons/` endpoint (#10534)
* Addons: return `ethicalads` data on `/_/addons/` endpoint
This data is consumed by the sponsorship addons to display an ad using
EthicalAds.
Related https://github.com/readthedocs/addons/issues/67
* Addons: return whether or not EthicalAd is enabled
* Lint: avoid `prospector` failing because missing import
`prospector` doesn't have access to this repository because it's private. So, we
are skipping the check for this line.
* Sponsorship: comment explaining the required setting