Unify API calls for Tree and Roadmap tabs within Epic
Summary
Currently when you visit Epic page, contents for Epic Tree tab (introduced with https://gitlab.com/gitlab-org/gitlab-ee/issues/10795) and Roadmap tab (introduced with https://gitlab.com/gitlab-org/gitlab-ee/issues/7327) are not rendered immediately, instead, when you click on Tab, a network request is made and then app is initialized and rendered.
Both Epic tree and Roadmap shown within Epic page use different APIs to fetch data, so to unify both, we first need address https://gitlab.com/gitlab-org/gitlab-ee/issues/12887 (which makes GraphQL common for both). Then we can do network request only once when page is loaded, and clicking tabs would only render UI where data is already available.
Edited by 🤖 GitLab Bot 🤖