View issue/merge request discussion thread quickly by loading in chunks

Description

• Loading an issue page may be slow under various scenarios (#23207 (closed)).
• The scope of this issue is to implement a solution to address #23207 (closed), in particular, the discussion thread section. If the discussion thread is very long, loading it takes a long time.
• Suggestions so far are pagination and/or infinite scrolling.

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.