Eliminate cached N+1 queries for projects in Issue API

In CE, every Issue entity is also a ProjectEntity, which calls entity&.project.try(:id) to show the project ID. In an API request with 100 issues, this would hit the Rails statement cache 100 times for the same project and cause unnecessary overhead as related models would also be loaded.

In EE, we call Issue#supports_weight? for each issue, which then calls project&.feature_available?(:issue_weights). If the project is not preloaded, this incurs additional overhead, as each individual Project object has to be queried. This can lead to a significant performance hit. In loading the CE project with 100 issues, this contributed to at least 22% of the load time.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/47031 for why testing this is a bit tricky.