Merge branch 'collapse-replies' into 'master'

Reduce size and padding of collapse replies button

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/173575



Merged-by: Andrew Fontaine <afontaine@gitlab.com>
Approved-by: Julia Miocene <jmiocene@gitlab.com>
Approved-by: Andrew Fontaine <afontaine@gitlab.com>
Co-authored-by: Annabel Dunstone Gray <annabel.dunstone@gmail.com>