View issue quickly by not loading everything

Description

• Loading an issue page may be slow under various scenarios (gitlab-ce#23207).
• The scope of this issue is to implement a solution to address gitlab-ce#23207 in part, by loading part of an issue at first, and then the rest subsequently (probably asynchronously).

Out of scope

• This issue is for navigating/linking to an issue directly. Linking to a specific comment is out of scope, and discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/24593.