Remote content link validation can exceed its max simultaneous fetch threads

[vsch]

The content cache for remote links is a project service instead of an application service. Remote links don't change for different projects.

The simultaneous fetch limit was enforced pre project. Each open project could potentially use another max fetch limit threads while updating the cache.

The content should be per application and fetch limit is also per application, which makes more sense.

Additionally, any pending fetch tasks should be cancelled when the cache is cleared.