Better UX for reply feature

Problem to solve

This is a spin off from https://gitlab.com/gitlab-org/gitlab-ce/issues/30299#note_155605159

As mentioned there (and in gitlab-ce#54238) the current reply function to comments can lead to confision for the users and that can lead to unstructured discussion threads with some comments on thread level and some unintended on the normal comment level.

Proposal

The proposal is to change the UI accordingly including, but not limited to

• Adding some more highlight to the reply area of a discussion thread so it gets recognized easier

This list can be discussed and also expanded with further suggestions.