Eliminate cached N+1 queries for projects in Issue API

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.