Do not access database from builds to check ad-free

[davidfischer]

There was an issue with the previous fix in #4329 which resulted in builders trying to access the database (project.gold_owners.exists makes a query). The builders don't have access to the database so this was a problem. Instead, this returns a show_advertising field on the API which checks both project.ad_free and project.gold_owners.exists.

The end result is that ad-free projects -- whether marked ad-free at the database level or because they have a Gold sponsor -- should not show the ad-block nag.

Fixes #4218

[humitos]

Changes look good.

I tried to build different projects locally and they did pass. Although, we don't have a simple way to replicate production environment locally (builder instance without access to the db)

[humitos]

Some tests for 3.6 failed because a download problem of elasticsearch. I re-triggerred that job only but should pass.