Add support for custom highlighting slash commands via warning flags

Kushal Pandyarequested to merge
ce-backport-3777-promote-to-epic into master

What does this MR do?

This MR is CE backport from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8051.

It adds support for custom highlighting for Slash command if it includes warning flag.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/3777

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